diff --git a/Core/API/ContactAPI.class.php b/Core/API/ContactAPI.class.php
deleted file mode 100644
index 59d863a..0000000
--- a/Core/API/ContactAPI.class.php
+++ /dev/null
@@ -1,291 +0,0 @@
-<?php
-
-namespace Core\API {
-
-  use Core\Objects\Context;
-
-  abstract class ContactAPI extends Request {
-
-    protected ?string $messageId;
-
-    public function __construct(Context $context, bool $externalCall, array $params) {
-      parent::__construct($context, $externalCall, $params);
-      $this->messageId = null;
-      $this->csrfTokenRequired = false;
-    }
-
-    protected function sendMail(string $name, ?string $fromEmail, string $subject, string $message, ?string $to = null): bool {
-      $request = new \Core\API\Mail\Send($this->context);
-      $this->success = $request->execute(array(
-        "subject" => $subject,
-        "body" => $message,
-        "replyTo" => $fromEmail,
-        "replyName" => $name,
-        "to" => $to
-      ));
-
-      $this->lastError = $request->getLastError();
-      if ($this->success) {
-        $this->messageId = $request->getResult()["messageId"];
-      }
-
-      return $this->success;
-    }
-  }
-}
-
-namespace Core\API\Contact {
-
-  use Core\API\ContactAPI;
-  use Core\API\Parameter\Parameter;
-  use Core\API\Parameter\StringType;
-  use Core\API\VerifyCaptcha;
-  use Core\Driver\SQL\Condition\Compare;
-  use Core\Driver\SQL\Condition\CondNot;
-  use Core\Driver\SQL\Expression\CaseWhen;
-  use Core\Driver\SQL\Expression\Sum;
-  use Core\Objects\Context;
-
-  class Request extends ContactAPI {
-
-    public function __construct(Context $context, bool $externalCall = false) {
-      $parameters = array(
-        'fromName' => new StringType('fromName', 32),
-        'fromEmail' => new Parameter('fromEmail', Parameter::TYPE_EMAIL),
-        'message' => new StringType('message', 512),
-      );
-
-      $settings = $context->getSettings();
-      if ($settings->isRecaptchaEnabled()) {
-        $parameters["captcha"] = new StringType("captcha");
-      }
-
-      parent::__construct($context, $externalCall, $parameters);
-    }
-
-    public function _execute(): bool {
-      $settings = $this->context->getSettings();
-      if ($settings->isRecaptchaEnabled()) {
-        $captcha = $this->getParam("captcha");
-        $req = new VerifyCaptcha($this->context);
-        if (!$req->execute(array("captcha" => $captcha, "action" => "contact"))) {
-          return $this->createError($req->getLastError());
-        }
-      }
-
-      // parameter
-      $message = $this->getParam("message");
-      $name = $this->getParam("fromName");
-      $email = $this->getParam("fromEmail");
-
-      $sendMail = $this->sendMail($name, $email, "Contact Request", $message);
-      $insertDB = $this->insertContactRequest();
-      if (!$sendMail && !$insertDB) {
-        return $this->createError("The contact request could not be sent. The Administrator is already informed. Please try again later.");
-      }
-
-      return $this->success;
-    }
-
-    private function insertContactRequest(): bool {
-      $sql = $this->context->getSQL();
-      $name = $this->getParam("fromName");
-      $email = $this->getParam("fromEmail");
-      $message = $this->getParam("message");
-      $messageId = $this->messageId ?? null;
-
-      $res = $sql->insert("ContactRequest", array("from_name", "from_email", "message", "messageId"))
-        ->addRow($name, $email, $message, $messageId)
-        ->returning("id")
-        ->execute();
-
-      $this->success = ($res !== FALSE);
-      $this->lastError = $sql->getLastError();
-      return $this->success;
-    }
-  }
-
-  class Respond extends ContactAPI {
-
-    public function __construct(Context $context, bool $externalCall = false) {
-      parent::__construct($context, $externalCall, array(
-        "requestId" => new Parameter("requestId", Parameter::TYPE_INT),
-        'message' => new StringType('message', 512),
-      ));
-      $this->loginRequired = true;
-    }
-
-    private function getSenderMail(): ?string {
-      $requestId = $this->getParam("requestId");
-      $sql = $this->context->getSQL();
-      $res = $sql->select("from_email")
-        ->from("ContactRequest")
-        ->where(new Compare("id", $requestId))
-        ->execute();
-
-      $this->success = ($res !== false);
-      $this->lastError = $sql->getLastError();
-
-      if ($this->success) {
-        if (empty($res)) {
-          return $this->createError("Request does not exist");
-        } else {
-          return $res[0]["from_email"];
-        }
-      }
-
-      return null;
-    }
-
-    private function insertResponseMessage(): bool {
-      $sql = $this->context->getSQL();
-      $message = $this->getParam("message");
-      $requestId = $this->getParam("requestId");
-
-      $this->success = $sql->insert("ContactMessage", ["request_id", "user_id", "message", "messageId", "read"])
-        ->addRow($requestId, $this->context->getUser()->getId(), $message, $this->messageId, true)
-        ->execute();
-
-      $this->lastError = $sql->getLastError();
-      return $this->success;
-    }
-
-    private function updateEntity() {
-      $sql = $this->context->getSQL();
-      $requestId = $this->getParam("requestId");
-
-      $sql->update("EntityLog")
-        ->set("modified", $sql->now())
-        ->where(new Compare("entityId", $requestId))
-        ->execute();
-    }
-
-    public function _execute(): bool {
-      $message = $this->getParam("message");
-      $senderMail = $this->getSenderMail();
-      if (!$this->success) {
-        return false;
-      }
-
-      $user = $this->context->getUser();
-      $fromName = $user->getUsername();
-      $fromEmail = $user->getEmail();
-
-      if (!$this->sendMail($fromName, $fromEmail, "Re: Contact Request", $message, $senderMail)) {
-        return false;
-      }
-
-      if (!$this->insertResponseMessage()) {
-        return false;
-      }
-
-      $this->updateEntity();
-      return $this->success;
-    }
-  }
-
-  class Fetch extends ContactAPI {
-
-    public function __construct(Context $context, bool $externalCall = false) {
-      parent::__construct($context, $externalCall, array());
-      $this->loginRequired = true;
-      $this->csrfTokenRequired = false;
-    }
-
-    public function _execute(): bool {
-      $sql = $this->context->getSQL();
-      $res = $sql->select("ContactRequest.id", "from_name", "from_email", "from_name",
-          new Sum(new CaseWhen(new CondNot("ContactMessage.read"), 1, 0), "unread"))
-        ->from("ContactRequest")
-        ->groupBy("ContactRequest.id")
-        ->leftJoin("ContactMessage", "ContactRequest.id", "ContactMessage.request_id")
-        ->execute();
-
-      $this->success = ($res !== false);
-      $this->lastError = $sql->getLastError();
-
-      if ($this->success) {
-        $this->result["contactRequests"] = [];
-        foreach ($res as $row) {
-          $this->result["contactRequests"][] = array(
-            "id" => intval($row["id"]),
-            "from_name" => $row["from_name"],
-            "from_email" => $row["from_email"],
-            "unread" => intval($row["unread"]),
-          );
-        }
-      }
-
-      return $this->success;
-    }
-  }
-
-  class Get extends ContactAPI {
-
-    public function __construct(Context $context, bool $externalCall = false) {
-      parent::__construct($context, $externalCall, array(
-        "requestId" => new Parameter("requestId", Parameter::TYPE_INT),
-      ));
-      $this->loginRequired = true;
-      $this->csrfTokenRequired = false;
-    }
-
-    private function updateRead() {
-      $requestId = $this->getParam("requestId");
-      $sql = $this->context->getSQL();
-      $sql->update("ContactMessage")
-        ->set("read", 1)
-        ->where(new Compare("request_id", $requestId))
-        ->execute();
-    }
-
-    public function _execute(): bool {
-      $requestId = $this->getParam("requestId");
-      $sql = $this->context->getSQL();
-
-      $res = $sql->select("from_name", "from_email", "message", "created_at")
-        ->from("ContactRequest")
-        ->where(new Compare("id", $requestId))
-        ->execute();
-
-      $this->success = ($res !== false);
-      $this->lastError = $sql->getLastError();
-
-      if ($this->success) {
-        if (empty($res)) {
-          return $this->createError("Request does not exist");
-        } else {
-          $row = $res[0];
-          $this->result["request"] = array(
-            "from_name" => $row["from_name"],
-            "from_email" => $row["from_email"],
-            "messages" => array(
-              ["sender_id" => null, "message" => $row["message"], "timestamp" => $row["created_at"]]
-            )
-          );
-
-          $res = $sql->select("user_id", "message", "created_at")
-            ->from("ContactMessage")
-            ->where(new Compare("request_id", $requestId))
-            ->orderBy("created_at")
-            ->execute();
-
-          $this->success = ($res !== false);
-          $this->lastError = $sql->getLastError();
-
-          if ($this->success) {
-            foreach ($res as $row) {
-              $this->result["request"]["messages"][] = array(
-                "sender_id" => $row["user_id"], "message" => $row["message"], "timestamp" => $row["created_at"]
-              );
-            }
-
-            $this->updateRead();
-          }
-        }
-      }
-
-      return $this->success;
-    }
-  }
-}
\ No newline at end of file
diff --git a/Core/API/GroupsAPI.class.php b/Core/API/GroupsAPI.class.php
index 7823482..2fa975a 100644
--- a/Core/API/GroupsAPI.class.php
+++ b/Core/API/GroupsAPI.class.php
@@ -31,6 +31,7 @@ namespace Core\API\Groups {
   use Core\API\Parameter\Parameter;
   use Core\API\Parameter\StringType;
   use Core\Objects\Context;
+  use Core\Objects\DatabaseEntity\Controller\NMRelation;
   use Core\Objects\DatabaseEntity\Group;
 
   class Fetch extends GroupsAPI {
@@ -46,20 +47,6 @@ namespace Core\API\Groups {
       $this->groupCount = 0;
     }
 
-    private function fetchGroupCount(): bool {
-
-      $sql = $this->context->getSQL();
-      $res = $sql->select($sql->count())->from("Group")->execute();
-      $this->success = ($res !== FALSE);
-      $this->lastError = $sql->getLastError();
-
-      if ($this->success) {
-        $this->groupCount = $res[0]["count"];
-      }
-
-      return $this->success;
-    }
-
     public function _execute(): bool {
       $page = $this->getParam("page");
       if($page < 1) {
@@ -71,39 +58,42 @@ namespace Core\API\Groups {
         return $this->createError("Invalid fetch count");
       }
 
-      if (!$this->fetchGroupCount()) {
-        return false;
+      $sql = $this->context->getSQL();
+      $groupCount = Group::count($sql);
+      if ($groupCount === false) {
+        return $this->createError("Error fetching group count: " . $sql->getLastError());
       }
 
-      $sql = $this->context->getSQL();
-      $res = $sql->select("Group.id as groupId", "Group.name as groupName", "Group.color as groupColor", $sql->count("UserGroup.user_id"))
-        ->from("Group")
-        ->leftJoin("UserGroup", "UserGroup.group_id", "Group.id")
-        ->groupBy("Group.id")
-        ->orderBy("Group.id")
+      $groups = Group::findBy(Group::createBuilder($sql, false)
+        ->orderBy("id")
         ->ascending()
         ->limit($count)
-        ->offset(($page - 1) * $count)
-        ->execute();
+        ->offset(($page - 1) * $count));
 
-      $this->success = ($res !== FALSE);
-      $this->lastError = $sql->getLastError();
-
-      if($this->success) {
-        $this->result["groups"] = array();
-        foreach($res as $row) {
-          $groupId = intval($row["groupId"]);
-          $groupName = $row["groupName"];
-          $groupColor = $row["groupColor"];
-          $memberCount = $row["usergroup_user_id_count"];
-          $this->result["groups"][$groupId] = array(
-            "name" => $groupName,
-            "memberCount" => $memberCount,
-            "color" => $groupColor,
-          );
-        }
+      if ($groups !== false) {
+        $this->result["groups"] = [];
         $this->result["pageCount"] = intval(ceil($this->groupCount / $count));
         $this->result["totalCount"] = $this->groupCount;
+
+        foreach ($groups as $groupId => $group) {
+          $this->result["groups"][$groupId] = $group->jsonSerialize();
+          $this->result["groups"][$groupId]["memberCount"] = 0;
+        }
+
+        $nmTable = NMRelation::buildTableName("User", "Group");
+        $res = $sql->select("group_id", $sql->count("user_id"))
+          ->from($nmTable)
+          ->groupBy("group_id")
+          ->execute();
+
+        if (is_array($res)) {
+          foreach ($res as $row) {
+            list ($groupId, $memberCount) = [$row["group_id"], $row["user_id_count"]];
+            if (isset($this->result["groups"][$groupId])) {
+              $this->result["groups"][$groupId]["memberCount"] = $memberCount;
+            }
+          }
+        }
       }
 
       return $this->success;
@@ -162,7 +152,7 @@ namespace Core\API\Groups {
 
     public function _execute(): bool {
       $id = $this->getParam("id");
-      if (in_array($id, DEFAULT_GROUPS)) {
+      if (in_array($id, array_keys(Group::GROUPS))) {
         return $this->createError("You cannot delete a default group.");
       }
 
diff --git a/Core/API/LogsAPI.class.php b/Core/API/LogsAPI.class.php
index afa05a2..3d58dd9 100644
--- a/Core/API/LogsAPI.class.php
+++ b/Core/API/LogsAPI.class.php
@@ -47,7 +47,8 @@ namespace Core\API\Logs {
         $shownLogLevels = array_slice(Logger::LOG_LEVELS, $logLevel);
       }
 
-      $query = SystemLog::findAllBuilder($sql)
+
+      $query = SystemLog::createBuilder($sql, false)
         ->orderBy("timestamp")
         ->descending();
 
@@ -59,7 +60,7 @@ namespace Core\API\Logs {
         $query->where(new CondIn(new Column("severity"), $shownLogLevels));
       }
 
-      $logEntries = $query->execute();
+      $logEntries = SystemLog::findBy($query);
       $this->success = $logEntries !== false;
       $this->lastError = $sql->getLastError();
 
diff --git a/Core/API/MailAPI.class.php b/Core/API/MailAPI.class.php
index 19365f3..c88c68c 100644
--- a/Core/API/MailAPI.class.php
+++ b/Core/API/MailAPI.class.php
@@ -46,6 +46,7 @@ namespace Core\API\Mail {
   use Core\API\MailAPI;
   use Core\API\Parameter\Parameter;
   use Core\API\Parameter\StringType;
+  use Core\Objects\DatabaseEntity\MailQueueItem;
   use DateTimeInterface;
   use Core\Driver\SQL\Column\Column;
   use Core\Driver\SQL\Condition\Compare;
@@ -84,7 +85,7 @@ namespace Core\API\Mail {
   class Send extends MailAPI {
     public function __construct(Context $context, $externalCall = false) {
       parent::__construct($context, $externalCall, array(
-        'to' => new Parameter('to', Parameter::TYPE_EMAIL, true, null),
+        'to' => new Parameter('to', Parameter::TYPE_EMAIL),
         'subject' => new StringType('subject', -1),
         'body' => new StringType('body', -1),
         'replyTo' => new Parameter('replyTo', Parameter::TYPE_EMAIL, true, null),
@@ -104,7 +105,7 @@ namespace Core\API\Mail {
 
       $fromMail = $mailConfig->getProperty('from');
       $mailFooter = $mailConfig->getProperty('mail_footer');
-      $toMail = $this->getParam('to') ?? $fromMail;
+      $toMail = $this->getParam('to');
       $subject = $this->getParam('subject');
       $replyTo = $this->getParam('replyTo');
       $replyName = $this->getParam('replyName');
@@ -119,10 +120,8 @@ namespace Core\API\Mail {
 
       if ($mailAsync) {
         $sql = $this->context->getSQL();
-        $this->success = $sql->insert("MailQueue", ["from", "to", "subject", "body",
-          "replyTo", "replyName", "gpgFingerprint"])
-          ->addRow($fromMail, $toMail, $subject, $body, $replyTo, $replyName, $gpgFingerprint)
-          ->execute() !== false;
+        $mailQueueItem = new MailQueueItem($fromMail, $toMail, $subject, $body, $replyTo, $replyName, $gpgFingerprint);
+        $this->success = $mailQueueItem->save($sql);
         $this->lastError = $sql->getLastError();
         return $this->success;
       }
@@ -223,77 +222,37 @@ namespace Core\API\Mail {
       }
 
       $sql = $this->context->getSQL();
-      $res = $sql->select("id", "from", "to", "subject", "body",
-        "replyTo", "replyName", "gpgFingerprint", "retryCount")
-        ->from("MailQueue")
+      $mailQueueItems = MailQueueItem::findBy(MailQueueItem::createBuilder($sql, false)
         ->where(new Compare("retryCount", 0, ">"))
         ->where(new Compare("status", "waiting"))
-        ->where(new Compare("nextTry", $sql->now(), "<="))
-        ->execute();
+        ->where(new Compare("nextTry", $sql->now(), "<=")));
 
-      $this->success = ($res !== false);
+      $this->success = ($mailQueueItems !== false);
       $this->lastError = $sql->getLastError();
 
-      if ($this->success && is_array($res)) {
+      if ($this->success && is_array($mailQueueItems)) {
         if ($debug) {
-          echo "Found " . count($res) . " mails to send" . PHP_EOL;
+          echo "Found " . count($mailQueueItems) . " mails to send" . PHP_EOL;
         }
 
-        $successfulMails = [];
-        foreach ($res as $row) {
+        $successfulMails = 0;
+        foreach ($mailQueueItems as $mailQueueItem) {
 
           if (time() - $startTime >= 45) {
             $this->lastError = "Not able to process whole mail queue within 45 seconds, will continue on next time";
             break;
           }
 
-          $to = $row["to"];
-          $subject = $row["subject"];
-
           if ($debug) {
-            echo "Sending subject=$subject to=$to" . PHP_EOL;
+            echo "Sending subject=$mailQueueItem->subject to=$mailQueueItem->to" . PHP_EOL;
           }
 
-          $mailId = intval($row["id"]);
-          $retryCount = intval($row["retryCount"]);
-          $req = new Send($this->context);
-          $args = [
-            "to" => $to,
-            "subject" => $subject,
-            "body" => $row["body"],
-            "replyTo" => $row["replyTo"],
-            "replyName" => $row["replyName"],
-            "gpgFingerprint" => $row["gpgFingerprint"],
-            "async" => false
-          ];
-          $success = $req->execute($args);
-          $error = $req->getLastError();
-
-          if (!$success) {
-            $delay = [0, 720, 360, 60, 30, 1];
-            $minutes = $delay[max(0, min(count($delay) - 1, $retryCount))];
-            $nextTry = (new \DateTime())->modify("+$minutes minute");
-            $sql->update("MailQueue")
-              ->set("retryCount", $retryCount - 1)
-              ->set("status", "error")
-              ->set("errorMessage", $error)
-              ->set("nextTry", $nextTry)
-              ->where(new Compare("id", $mailId))
-              ->execute();
-          } else {
-            $successfulMails[] = $mailId;
+          if ($mailQueueItem->send($this->context)) {
+            $successfulMails++;
           }
         }
 
-        $this->success = count($successfulMails) === count($res);
-        if (!empty($successfulMails)) {
-          $res = $sql->update("MailQueue")
-            ->set("status", "success")
-            ->where(new CondIn(new Column("id"), $successfulMails))
-            ->execute();
-          $this->success = $res !== false;
-          $this->lastError = $sql->getLastError();
-        }
+        $this->success = $successfulMails === count($mailQueueItems);
       }
 
       return $this->success;
diff --git a/Core/API/NewsAPI.class.php b/Core/API/NewsAPI.class.php
index 9d889fd..5332176 100644
--- a/Core/API/NewsAPI.class.php
+++ b/Core/API/NewsAPI.class.php
@@ -19,6 +19,7 @@ namespace Core\API\News {
   use Core\API\Parameter\StringType;
   use Core\Driver\SQL\Condition\Compare;
   use Core\Objects\Context;
+  use Core\Objects\DatabaseEntity\Group;
   use Core\Objects\DatabaseEntity\News;
 
   class Get extends NewsAPI {
@@ -40,7 +41,7 @@ namespace Core\API\News {
       }
 
       $sql = $this->context->getSQL();
-      $newsQuery = News::findAllBuilder($sql)
+      $newsQuery = News::createBuilder($sql, false)
         ->limit($limit)
         ->orderBy("published_at")
         ->descending()
@@ -50,7 +51,7 @@ namespace Core\API\News {
         $newsQuery->where(new Compare("published_at", $since, ">="));
       }
 
-      $newsArray = $newsQuery->execute();
+      $newsArray = News::findBy($newsQuery);
       $this->success = $newsArray !== null;
       $this->lastError = $sql->getLastError();
 
@@ -113,7 +114,7 @@ namespace Core\API\News {
         return false;
       } else if ($news === null) {
         return $this->createError("News Post not found");
-      } else if ($news->publishedBy->getId() !== $currentUser->getId() && !$currentUser->hasGroup(USER_GROUP_ADMIN)) {
+      } else if ($news->publishedBy->getId() !== $currentUser->getId() && !$currentUser->hasGroup(Group::ADMIN)) {
         return $this->createError("You do not have permissions to delete news post of other users.");
       }
 
@@ -144,7 +145,7 @@ namespace Core\API\News {
         return false;
       } else if ($news === null) {
         return $this->createError("News Post not found");
-      } else if ($news->publishedBy->getId() !== $currentUser->getId() && !$currentUser->hasGroup(USER_GROUP_ADMIN)) {
+      } else if ($news->publishedBy->getId() !== $currentUser->getId() && !$currentUser->hasGroup(Group::ADMIN)) {
         return $this->createError("You do not have permissions to edit news post of other users.");
       }
 
diff --git a/Core/API/NotificationsAPI.class.php b/Core/API/NotificationsAPI.class.php
deleted file mode 100644
index 4135401..0000000
--- a/Core/API/NotificationsAPI.class.php
+++ /dev/null
@@ -1,231 +0,0 @@
-<?php
-
-namespace Core\API {
-
-  use Core\Objects\Context;
-
-  abstract class NotificationsAPI extends Request {
-    public function __construct(Context $context, bool $externalCall = false, array $params = array()) {
-      parent::__construct($context, $externalCall, $params);
-    }
-  }
-}
-
-namespace Core\API\Notifications {
-
-  use Core\API\NotificationsAPI;
-  use Core\API\Parameter\Parameter;
-  use Core\API\Parameter\StringType;
-  use Core\Driver\SQL\Column\Column;
-  use Core\Driver\SQL\Condition\Compare;
-  use Core\Driver\SQL\Condition\CondIn;
-  use Core\Driver\SQL\Query\Select;
-  use Core\Objects\Context;
-  use Core\Objects\DatabaseEntity\Group;
-  use Core\Objects\DatabaseEntity\Notification;
-  use Core\Objects\DatabaseEntity\User;
-
-  class Create extends NotificationsAPI {
-
-    public function __construct(Context $context, $externalCall = false) {
-      parent::__construct($context, $externalCall, array(
-        'groupId' => new Parameter('groupId', Parameter::TYPE_INT, true),
-        'userId' => new Parameter('userId', Parameter::TYPE_INT, true),
-        'title' =>  new StringType('title', 32),
-        'message' =>  new StringType('message', 256),
-      ));
-      $this->isPublic = false;
-    }
-
-    private function insertUserNotification($userId, $notificationId): bool {
-      $sql = $this->context->getSQL();
-      $res = $sql->insert("UserNotification", array("user_id", "notification_id"))
-        ->addRow($userId, $notificationId)
-        ->execute();
-
-      $this->success = ($res !== FALSE);
-      $this->lastError = $sql->getLastError();
-      return $this->success;
-    }
-
-    private function insertGroupNotification($groupId, $notificationId): bool {
-      $sql = $this->context->getSQL();
-      $res = $sql->insert("GroupNotification", array("group_id", "notification_id"))
-        ->addRow($groupId, $notificationId)
-        ->execute();
-
-      $this->success = ($res !== FALSE);
-      $this->lastError = $sql->getLastError();
-      return $this->success;
-    }
-
-    private function createNotification($title, $message): bool|int {
-      $sql = $this->context->getSQL();
-      $notification = new Notification();
-      $notification->title = $title;
-      $notification->message = $message;
-
-      $this->success = ($notification->save($sql) !== FALSE);
-      $this->lastError = $sql->getLastError();
-
-      if ($this->success) {
-        return $notification->getId();
-      }
-
-      return $this->success;
-    }
-
-    public function _execute(): bool {
-      $sql = $this->context->getSQL();
-      $userId = $this->getParam("userId");
-      $groupId = $this->getParam("groupId");
-      $title = $this->getParam("title");
-      $message = $this->getParam("message");
-
-      if (is_null($userId) && is_null($groupId)) {
-        return $this->createError("Either userId or groupId must be specified.");
-      } else if(!is_null($userId) && !is_null($groupId)) {
-        return $this->createError("Only one of userId and groupId must be specified.");
-      } else if(!is_null($userId)) {
-        if (User::exists($sql, $userId)) {
-          $id = $this->createNotification($title, $message);
-          if ($this->success) {
-            return $this->insertUserNotification($userId, $id);
-          }
-        } else {
-          return $this->createError("User not found: $userId");
-        }
-      } else if(!is_null($groupId)) {
-        if (Group::exists($sql, $groupId)) {
-          $id = $this->createNotification($title, $message);
-          if ($this->success) {
-            return $this->insertGroupNotification($groupId, $id);
-          }
-        } else {
-          return $this->createError("Group not found: $groupId");
-        }
-      }
-
-      return $this->success;
-    }
-  }
-
-  class Fetch extends NotificationsAPI {
-
-    private array $notifications;
-    private array $notificationIds;
-
-    public function __construct(Context $context, $externalCall = false) {
-      parent::__construct($context, $externalCall, array(
-        'new' => new Parameter('new', Parameter::TYPE_BOOLEAN, true, true)
-      ));
-      $this->loginRequired = true;
-    }
-
-    private function fetchUserNotifications(): bool {
-      $onlyNew = $this->getParam('new');
-      $userId = $this->context->getUser()->getId();
-      $sql = $this->context->getSQL();
-      $query = $sql->select($sql->distinct("Notification.id"), "created_at", "title", "message", "type")
-        ->from("Notification")
-        ->innerJoin("UserNotification", "UserNotification.notification_id", "Notification.id")
-        ->where(new Compare("UserNotification.user_id", $userId))
-        ->orderBy("created_at")->descending();
-
-      if ($onlyNew) {
-        $query->where(new Compare("UserNotification.seen", false));
-      }
-
-      return $this->fetchNotifications($query);
-    }
-
-    private function fetchGroupNotifications(): bool {
-      $onlyNew = $this->getParam('new');
-      $userId = $this->context->getUser()->getId();
-      $sql = $this->context->getSQL();
-      $query = $sql->select($sql->distinct("Notification.id"), "created_at", "title", "message", "type")
-        ->from("Notification")
-        ->innerJoin("GroupNotification", "GroupNotification.notification_id", "Notification.id")
-        ->innerJoin("UserGroup", "GroupNotification.group_id", "UserGroup.group_id")
-        ->where(new Compare("UserGroup.user_id", $userId))
-        ->orderBy("created_at")->descending();
-
-      if ($onlyNew) {
-        $query->where(new Compare("GroupNotification.seen", false));
-      }
-
-      return $this->fetchNotifications($query);
-    }
-
-    private function fetchNotifications(Select $query): bool {
-      $sql = $this->context->getSQL();
-      $res = $query->execute();
-      $this->success = ($res !== FALSE);
-      $this->lastError = $sql->getLastError();
-
-      if ($this->success) {
-        foreach($res as $row) {
-          $id = $row["id"];
-          if (!in_array($id, $this->notificationIds)) {
-            $this->notificationIds[] = $id;
-            $this->notifications[] = array(
-              "id" => $id,
-              "title" => $row["title"],
-              "message" => $row["message"],
-              "created_at" => $row["created_at"],
-              "type" => $row["type"]
-            );
-          }
-        }
-      }
-
-      return $this->success;
-    }
-
-    public function _execute(): bool {
-      $this->notifications = array();
-      $this->notificationIds = array();
-      if ($this->fetchUserNotifications() && $this->fetchGroupNotifications()) {
-        $this->result["notifications"] = $this->notifications;
-      }
-
-      return $this->success;
-    }
-  }
-
-  class Seen extends NotificationsAPI {
-
-    public function __construct(Context $context, bool $externalCall = false) {
-      parent::__construct($context, $externalCall, array());
-      $this->loginRequired = true;
-    }
-
-    public function _execute(): bool {
-
-      $currentUser = $this->context->getUser();
-      $sql = $this->context->getSQL();
-      $res = $sql->update("UserNotification")
-        ->set("seen", true)
-        ->where(new Compare("user_id", $currentUser->getId()))
-        ->execute();
-
-      $this->success = ($res !== FALSE);
-      $this->lastError = $sql->getLastError();
-
-      if ($this->success) {
-        $res = $sql->update("GroupNotification")
-          ->set("seen", true)
-          ->where(new CondIn(new Column("group_id"),
-            $sql->select("group_id")
-              ->from("UserGroup")
-              ->where(new Compare("user_id", $currentUser->getId()))))
-          ->execute();
-
-        $this->success = ($res !== FALSE);
-        $this->lastError = $sql->getLastError();
-      }
-
-      return $this->success;
-    }
-  }
-}
\ No newline at end of file
diff --git a/Core/API/PermissionAPI.class.php b/Core/API/PermissionAPI.class.php
index 6821bb1..d6ae29f 100644
--- a/Core/API/PermissionAPI.class.php
+++ b/Core/API/PermissionAPI.class.php
@@ -3,6 +3,7 @@
 namespace Core\API {
 
   use Core\Objects\Context;
+  use Core\Objects\DatabaseEntity\Group;
 
   abstract class PermissionAPI extends Request {
 
@@ -12,7 +13,7 @@ namespace Core\API {
 
     protected function checkStaticPermission(): bool {
       $user = $this->context->getUser();
-      if (!$user || !$user->hasGroup(USER_GROUP_ADMIN)) {
+      if (!$user || !$user->hasGroup(Group::ADMIN)) {
         return $this->createError("Permission denied.");
       }
 
@@ -34,7 +35,6 @@ namespace Core\API\Permission {
   use Core\Driver\SQL\Strategy\UpdateStrategy;
   use Core\Objects\Context;
   use Core\Objects\DatabaseEntity\Group;
-  use Core\Objects\DatabaseEntity\User;
 
   class Check extends PermissionAPI {
 
diff --git a/Core/API/Swagger.class.php b/Core/API/Swagger.class.php
index 03ccc2b..b423714 100644
--- a/Core/API/Swagger.class.php
+++ b/Core/API/Swagger.class.php
@@ -4,6 +4,7 @@ namespace Core\API;
 
 use Core\API\Parameter\StringType;
 use Core\Objects\Context;
+use Core\Objects\DatabaseEntity\Group;
 use Core\Objects\DatabaseEntity\User;
 
 class Swagger extends Request {
@@ -90,7 +91,7 @@ class Swagger extends Request {
     }
 
     // special case: hardcoded permission
-    if ($request instanceof Permission\Save && (!$currentUser || !$currentUser->hasGroup(USER_GROUP_ADMIN))) {
+    if ($request instanceof Permission\Save && (!$currentUser || !$currentUser->hasGroup(Group::ADMIN))) {
       return false;
     }
 
diff --git a/Core/API/UserAPI.class.php b/Core/API/UserAPI.class.php
index 4051414..9841dc4 100644
--- a/Core/API/UserAPI.class.php
+++ b/Core/API/UserAPI.class.php
@@ -75,7 +75,7 @@ namespace Core\API {
         $this->checkPasswordRequirements($password, $confirmPassword);
     }
 
-    protected function insertUser($username, $email, $password, $confirmed, $fullName = ""): bool|User {
+    protected function insertUser(string $username, ?string $email, string $password, bool $confirmed, string $fullName = "", array $groups = []): bool|User {
       $sql = $this->context->getSQL();
 
       $user = new User();
@@ -86,6 +86,7 @@ namespace Core\API {
       $user->email = $email;
       $user->confirmed = $confirmed;
       $user->fullName = $fullName ?? "";
+      $user->groups = $groups;
 
       $this->success = ($user->save($sql) !== FALSE);
       $this->lastError = $sql->getLastError();
@@ -107,12 +108,11 @@ namespace Core\API {
 
     protected function checkToken(string $token) : UserToken|bool {
       $sql = $this->context->getSQL();
-      $userToken = UserToken::findBuilder($sql)
+      $userToken = UserToken::findBy(UserToken::createBuilder($sql, true)
         ->where(new Compare("UserToken.token", $token))
         ->where(new Compare("UserToken.valid_until", $sql->now(), ">"))
         ->where(new Compare("UserToken.used", 0))
-        ->fetchEntities()
-        ->execute();
+        ->fetchEntities());
 
       if ($userToken === false) {
         return $this->createError("Error verifying token: " . $sql->getLastError());
@@ -128,11 +128,16 @@ namespace Core\API {
 
 namespace Core\API\User {
 
+  use Core\API\Parameter\ArrayType;
   use Core\API\Parameter\Parameter;
   use Core\API\Parameter\StringType;
   use Core\API\Template\Render;
   use Core\API\UserAPI;
   use Core\API\VerifyCaptcha;
+  use Core\Driver\SQL\Condition\CondBool;
+  use Core\Driver\SQL\Condition\CondNot;
+  use Core\Driver\SQL\Condition\CondOr;
+  use Core\Objects\DatabaseEntity\Group;
   use Core\Objects\DatabaseEntity\UserToken;
   use Core\Driver\SQL\Column\Column;
   use Core\Driver\SQL\Condition\Compare;
@@ -146,12 +151,15 @@ namespace Core\API\User {
 
   class Create extends UserAPI {
 
+    private User $user;
+
     public function __construct(Context $context, $externalCall = false) {
       parent::__construct($context, $externalCall, array(
         'username' => new StringType('username', 32),
         'email' => new Parameter('email', Parameter::TYPE_EMAIL, true, NULL),
         'password' => new StringType('password'),
         'confirmPassword' => new StringType('confirmPassword'),
+        'groups' => new ArrayType("groups", Parameter::TYPE_INT, true, true, [])
       ));
 
       $this->loginRequired = true;
@@ -171,21 +179,36 @@ namespace Core\API\User {
         return false;
       }
 
+      $groups = [];
+      $sql = $this->context->getSQL();
+      $requestedGroups = array_unique($this->getParam("groups"));
+      if (!empty($requestedGroups)) {
+        $groups = Group::findAll($sql, new CondIn(new Column("id"), $requestedGroups));
+        foreach ($requestedGroups as $groupId) {
+          if (!isset($groups[$groupId])) {
+            return $this->createError("Group with id=$groupId does not exist.");
+          }
+        }
+      }
+
       // prevent duplicate keys
       $email = (!is_null($email) && empty($email)) ? null : $email;
-      $user = $this->insertUser($username, $email, $password, true);
+      $user = $this->insertUser($username, $email, $password, true, "", $groups);
       if ($user !== false) {
+        $this->user = $user;
         $this->result["userId"] = $user->getId();
       }
 
       return $this->success;
     }
+
+    public function getUser(): User {
+      return $this->user;
+    }
   }
 
   class Fetch extends UserAPI {
 
-    private int $userCount;
-
     public function __construct(Context $context, $externalCall = false) {
       parent::__construct($context, $externalCall, array(
         'page' => new Parameter('page', Parameter::TYPE_INT, true, 1),
@@ -193,21 +216,7 @@ namespace Core\API\User {
       ));
     }
 
-    private function getUserCount(): bool {
-
-      $sql = $this->context->getSQL();
-      $res = $sql->select($sql->count())->from("User")->execute();
-      $this->success = ($res !== FALSE);
-      $this->lastError = $sql->getLastError();
-
-      if ($this->success) {
-        $this->userCount = $res[0]["count"];
-      }
-
-      return $this->success;
-    }
-
-    private function selectIds($page, $count) {
+    private function selectIds($page, $count): array|bool {
       $sql = $this->context->getSQL();
       $res = $sql->select("User.id")
         ->from("User")
@@ -241,72 +250,57 @@ namespace Core\API\User {
         return $this->createError("Invalid fetch count");
       }
 
-      if (!$this->getUserCount()) {
-        return false;
-      }
-
-      $userIds = $this->selectIds($page, $count);
-      if ($userIds === false) {
-        return false;
+      $condition = null;
+      $currentUser = $this->context->getUser();
+      $fullInfo = ($currentUser->hasGroup(Group::ADMIN) ||
+                   $currentUser->hasGroup(Group::SUPPORT));
+      if (!$fullInfo) {
+        $condition = new CondOr(
+          new Compare("User.id", $currentUser->getId()),
+          new CondBool("User.confirmed")
+        );
       }
 
       $sql = $this->context->getSQL();
-      $res = $sql->select("User.id as userId", "User.name", "User.email", "User.registered_at", "User.confirmed",
-        "User.profile_picture", "User.full_name", "Group.id as groupId", "User.last_online",
-        "Group.name as groupName", "Group.color as groupColor")
-        ->from("User")
-        ->leftJoin("UserGroup", "User.id", "UserGroup.user_id")
-        ->leftJoin("Group", "Group.id", "UserGroup.group_id")
-        ->where(new CondIn(new Column("User.id"), $userIds))
-        ->execute();
+      $userCount = User::count($sql, $condition);
+      if ($userCount === false) {
+        return $this->createError("Error fetching user count: " . $sql->getLastError());
+      }
 
-      $this->success = ($res !== FALSE);
-      $this->lastError = $sql->getLastError();
-      $currentUser = $this->context->getUser();
+      $userQuery = User::createBuilder($sql, false)
+        ->orderBy("id")
+        ->ascending()
+        ->limit($count)
+        ->offset(($page - 1) * $count)
+        ->fetchEntities();
 
-      if ($this->success) {
-        $this->result["users"] = array();
-        foreach ($res as $row) {
-          $userId = intval($row["userId"]);
-          $groupId = $row["groupId"];
-          $groupName = $row["groupName"];
-          $groupColor = $row["groupColor"];
+      if ($condition) {
+        $userQuery->where($condition);
+      }
 
-          $fullInfo = ($userId === $currentUser->getId() ||
-            $currentUser->hasGroup(USER_GROUP_ADMIN) ||
-            $currentUser->hasGroup(USER_GROUP_SUPPORT));
+      $users = User::findBy($userQuery);
+      if ($users !== false) {
+        $this->result["users"] = [];
 
-          if (!isset($this->result["users"][$userId])) {
-            $user = array(
-              "id" => $userId,
-              "name" => $row["name"],
-              "fullName" => $row["full_name"],
-              "profilePicture" => $row["profile_picture"],
-              "email" => $row["email"],
-              "confirmed" => $sql->parseBool($row["confirmed"]),
-              "groups" => array(),
-            );
+        foreach ($users as $userId => $user) {
+          $serialized = $user->jsonSerialize();
 
-            if ($fullInfo) {
-              $user["registered_at"] = $row["registered_at"];
-              $user["last_online"] = $row["last_online"];
-            } else if (!$sql->parseBool($row["confirmed"])) {
-              continue;
+          if (!$fullInfo && $userId !== $currentUser->getId()) {
+            $publicAttributes = ["id", "name", "fullName", "profilePicture", "email", "groups"];
+            foreach (array_keys($serialized) as $attr) {
+              if (!in_array($attr, $publicAttributes)) {
+                unset($serialized[$attr]);
+              }
             }
-
-            $this->result["users"][$userId] = $user;
           }
 
-          if (!is_null($groupId)) {
-            $this->result["users"][$userId]["groups"][intval($groupId)] = array(
-              "id" => intval($groupId),
-              "name" => $groupName,
-              "color" => $groupColor
-            );
-          }
+          $this->result["users"][$userId] = $serialized;
         }
+
         $this->result["pageCount"] = intval(ceil($this->userCount / $count));
         $this->result["totalCount"] = $this->userCount;
+      } else {
+        return $this->createError("Error fetching users: " . $sql->getLastError());
       }
 
       return $this->success;
@@ -338,13 +332,13 @@ namespace Core\API\User {
         // either we are querying own info or we are support / admin
         $currentUser = $this->context->getUser();
         $canView = ($userId === $currentUser->getId() ||
-          $currentUser->hasGroup(USER_GROUP_ADMIN) ||
-          $currentUser->hasGroup(USER_GROUP_SUPPORT));
+          $currentUser->hasGroup(Group::ADMIN) ||
+          $currentUser->hasGroup(Group::SUPPORT));
 
         // full info only when we have administrative privileges, or we are querying ourselves
         $fullInfo = ($userId === $currentUser->getId() ||
-          $currentUser->hasGroup(USER_GROUP_ADMIN) ||
-          $currentUser->hasGroup(USER_GROUP_SUPPORT));
+          $currentUser->hasGroup(Group::ADMIN) ||
+          $currentUser->hasGroup(Group::SUPPORT));
 
         if (!$canView) {
 
@@ -617,10 +611,9 @@ namespace Core\API\User {
       $stayLoggedIn = $this->getParam('stayLoggedIn');
 
       $sql = $this->context->getSQL();
-      $user = User::findBuilder($sql)
+      $user = User::findBy(User::createBuilder($sql, true)
         ->where(new Compare("User.name", $username), new Compare("User.email", $username))
-        ->fetchEntities()
-        ->execute();
+        ->fetchEntities());
 
       if ($user !== false) {
         if ($user === null) {
@@ -842,7 +835,7 @@ namespace Core\API\User {
             $groupIds[] = $param->value;
           }
 
-          if ($id === $currentUser->getId() && !in_array(USER_GROUP_ADMIN, $groupIds)) {
+          if ($id === $currentUser->getId() && !in_array(Group::ADMIN, $groupIds)) {
             return $this->createError("Cannot remove Administrator group from own user.");
           }
         }
@@ -958,10 +951,9 @@ namespace Core\API\User {
 
       $sql = $this->context->getSQL();
       $email = $this->getParam("email");
-      $user = User::findBuilder($sql)
+      $user = User::findBy(User::createBuilder($sql, true)
         ->where(new Compare("email", $email))
-        ->fetchEntities()
-        ->execute();
+        ->fetchEntities());
       if ($user === false) {
         return $this->createError("Could not fetch user details: " . $sql->getLastError());
       } else if ($user !== null) {
@@ -1040,10 +1032,9 @@ namespace Core\API\User {
 
       $email = $this->getParam("email");
       $sql = $this->context->getSQL();
-      $user = User::findBuilder($sql)
+      $user = User::findBy(User::createBuilder($sql, true)
         ->where(new Compare("User.email", $email))
-        ->where(new Compare("User.confirmed", false))
-        ->execute();
+        ->where(new Compare("User.confirmed", false)));
 
       if ($user === false) {
         return $this->createError("Error retrieving user details: " . $sql->getLastError());
@@ -1052,11 +1043,10 @@ namespace Core\API\User {
         return true;
       }
 
-      $userToken = UserToken::findBuilder($sql)
+      $userToken = UserToken::findBy(UserToken::createBuilder($sql, true)
         ->where(new Compare("used", false))
         ->where(new Compare("tokenType", UserToken::TYPE_EMAIL_CONFIRM))
-        ->where(new Compare("user_id", $user->getId()))
-        ->execute();
+        ->where(new Compare("user_id", $user->getId())));
 
       $validHours = 48;
       if ($userToken === false) {
@@ -1366,12 +1356,11 @@ namespace Core\API\User {
       $token = $this->getParam("token");
       $sql = $this->context->getSQL();
 
-      $userToken = UserToken::findBuilder($sql)
+      $userToken = UserToken::findBy(UserToken::createBuilder($sql, true)
         ->where(new Compare("token", $token))
         ->where(new Compare("valid_until", $sql->now(), ">="))
         ->where(new Compare("user_id", $currentUser->getId()))
-        ->where(new Compare("token_type", UserToken::TYPE_GPG_CONFIRM))
-        ->execute();
+        ->where(new Compare("token_type", UserToken::TYPE_GPG_CONFIRM)));
 
       if ($userToken !== false) {
         if ($userToken === null) {
diff --git a/Core/Configuration/CreateDatabase.class.php b/Core/Configuration/CreateDatabase.class.php
index d91390b..83cc889 100644
--- a/Core/Configuration/CreateDatabase.class.php
+++ b/Core/Configuration/CreateDatabase.class.php
@@ -3,9 +3,9 @@
 namespace Core\Configuration;
 
 use Core\Driver\SQL\SQL;
-use Core\Driver\SQL\Strategy\SetNullStrategy;
-use Core\Driver\SQL\Strategy\CascadeStrategy;
-use Core\Objects\DatabaseEntity\DatabaseEntity;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
+use Core\Objects\DatabaseEntity\Group;
+use Core\Objects\DatabaseEntity\Language;
 use PHPUnit\Util\Exception;
 
 class CreateDatabase extends DatabaseScript {
@@ -15,37 +15,16 @@ class CreateDatabase extends DatabaseScript {
 
     self::loadEntities($queries, $sql);
 
-    $queries[] = $sql->insert("Language", array("code", "name"))
-      ->addRow("en_US", 'American English')
-      ->addRow("de_DE", 'Deutsch Standard');
+    $queries[] = Language::getHandler($sql)->getInsertQuery([
+      new Language(Language::AMERICAN_ENGLISH, "en_US", 'American English'),
+      new Language(Language::AMERICAN_ENGLISH, "de_DE", 'Deutsch Standard'),
+    ]);
 
-    $queries[] = $sql->insert("Group", array("name", "color"))
-      ->addRow(USER_GROUP_MODERATOR_NAME, "#007bff")
-      ->addRow(USER_GROUP_SUPPORT_NAME, "#28a745")
-      ->addRow(USER_GROUP_ADMIN_NAME, "#dc3545");
-
-    $queries[] = $sql->createTable("UserGroup")
-      ->addInt("user_id")
-      ->addInt("group_id")
-      ->unique("user_id", "group_id")
-      ->foreignKey("user_id", "User", "id", new CascadeStrategy())
-      ->foreignKey("group_id", "Group", "id", new CascadeStrategy());
-
-    $queries[] = $sql->createTable("UserNotification")
-      ->addInt("user_id")
-      ->addInt("notification_id")
-      ->addBool("seen", false)
-      ->foreignKey("user_id", "User", "id")
-      ->foreignKey("notification_id", "Notification", "id")
-      ->unique("user_id", "notification_id");
-
-    $queries[] = $sql->createTable("GroupNotification")
-      ->addInt("group_id")
-      ->addInt("notification_id")
-      ->addBool("seen", false)
-      ->foreignKey("group_id", "Group", "id")
-      ->foreignKey("notification_id", "Notification", "id")
-      ->unique("group_id", "notification_id");
+    $queries[] = Group::getHandler($sql)->getInsertQuery([
+      new Group(Group::ADMIN, Group::GROUPS[Group::ADMIN], "#007bff"),
+      new Group(Group::MODERATOR, Group::GROUPS[Group::MODERATOR], "#28a745"),
+      new Group(Group::SUPPORT, Group::GROUPS[Group::SUPPORT], "#dc3545"),
+    ]);
 
     $queries[] = $sql->createTable("Visitor")
       ->addInt("day")
@@ -81,92 +60,43 @@ class CreateDatabase extends DatabaseScript {
       ->addBool("private", false) // these values are not returned from '/api/settings/get', but can be changed
       ->addBool("readonly", false) // these values are neither returned, nor can be changed from outside
       ->primaryKey("name");
-
-    $settingsQuery = $sql->insert("Settings", array("name", "value", "private", "readonly"))
-      // ->addRow("mail_enabled", "0") # this key will be set during installation
-      ->addRow("mail_host", "", false, false)
-      ->addRow("mail_port", "", false, false)
-      ->addRow("mail_username", "", false, false)
-      ->addRow("mail_password", "", true, false)
-      ->addRow("mail_from", "", false, false)
-      ->addRow("mail_last_sync", "", false, false)
-      ->addRow("mail_footer", "", false, false);
-
+    $settingsQuery = $sql->insert("Settings", array("name", "value", "private", "readonly"));
     (Settings::loadDefaults())->addRows($settingsQuery);
     $queries[] = $settingsQuery;
 
-    $queries[] = $sql->createTable("ContactRequest")
-      ->addSerial("id")
-      ->addString("from_name", 32)
-      ->addString("from_email", 64)
-      ->addString("message", 512)
-      ->addString("messageId", 78, true) # null = don't sync with mails (usually if mail could not be sent)
-      ->addDateTime("created_at", false, $sql->currentTimestamp())
-      ->unique("messageId")
-      ->primaryKey("id");
-
-    $queries[] = $sql->createTable("ContactMessage")
-      ->addSerial("id")
-      ->addInt("request_id")
-      ->addInt("user_id", true) # null = customer has sent this message
-      ->addString("message", 512)
-      ->addString("messageId", 78)
-      ->addDateTime("created_at", false, $sql->currentTimestamp())
-      ->addBool("read", false)
-      ->unique("messageId")
-      ->primaryKey("id")
-      ->foreignKey("request_id", "ContactRequest", "id", new CascadeStrategy())
-      ->foreignKey("user_id", "User", "id", new SetNullStrategy());
-
     $queries[] = $sql->createTable("ApiPermission")
       ->addString("method", 32)
       ->addJson("groups", true, '[]')
       ->addString("description", 128, false, "")
       ->primaryKey("method");
 
-    $queries[] = $sql->createTable("MailQueue")
-      ->addSerial("id")
-      ->addString("from", 64)
-      ->addString("to", 64)
-      ->addString("subject")
-      ->addString("body")
-      ->addString("replyTo", 64, true)
-      ->addString("replyName", 32, true)
-      ->addString("gpgFingerprint", 64, true)
-      ->addEnum("status", ["waiting", "success", "error"], false, 'waiting')
-      ->addInt("retryCount", false, 5)
-      ->addDateTime("nextTry", false, $sql->now())
-      ->addString("errorMessage", NULL, true)
-      ->primaryKey("id");
-
-    $queries = array_merge($queries, \Core\Configuration\Patch\EntityLog_2021_04_08::createTableLog($sql, "MailQueue", 30));
-
     $queries[] = $sql->insert("ApiPermission", array("method", "groups", "description"))
       ->addRow("ApiKey/create", array(), "Allows users to create API-Keys for themselves")
       ->addRow("ApiKey/fetch", array(), "Allows users to list their API-Keys")
       ->addRow("ApiKey/refresh", array(), "Allows users to refresh their API-Keys")
       ->addRow("ApiKey/revoke", array(), "Allows users to revoke their API-Keys")
-      ->addRow("Groups/fetch", array(USER_GROUP_SUPPORT, USER_GROUP_ADMIN), "Allows users to list all available groups")
-      ->addRow("Groups/create", array(USER_GROUP_ADMIN), "Allows users to create a new groups")
-      ->addRow("Groups/delete", array(USER_GROUP_ADMIN), "Allows users to delete a group")
-      ->addRow("Routes/fetch", array(USER_GROUP_ADMIN), "Allows users to list all configured routes")
-      ->addRow("Routes/save", array(USER_GROUP_ADMIN), "Allows users to create, delete and modify routes")
-      ->addRow("Mail/test", array(USER_GROUP_SUPPORT, USER_GROUP_ADMIN), "Allows users to send a test email to a given address")
-      ->addRow("Mail/Sync", array(USER_GROUP_SUPPORT, USER_GROUP_ADMIN), "Allows users to synchronize mails with the database")
-      ->addRow("Settings/get", array(USER_GROUP_ADMIN), "Allows users to fetch server settings")
-      ->addRow("Settings/set", array(USER_GROUP_ADMIN), "Allows users create, delete or modify server settings")
-      ->addRow("Stats", array(USER_GROUP_ADMIN, USER_GROUP_SUPPORT), "Allows users to fetch server stats")
-      ->addRow("User/create", array(USER_GROUP_ADMIN), "Allows users to create a new user, email address does not need to be confirmed")
-      ->addRow("User/fetch", array(USER_GROUP_ADMIN, USER_GROUP_SUPPORT), "Allows users to list all registered users")
-      ->addRow("User/get", array(USER_GROUP_ADMIN, USER_GROUP_SUPPORT), "Allows users to get information about a single user")
-      ->addRow("User/invite", array(USER_GROUP_ADMIN), "Allows users to create a new user and send them an invitation link")
-      ->addRow("User/edit", array(USER_GROUP_ADMIN), "Allows users to edit details and group memberships of any user")
-      ->addRow("User/delete", array(USER_GROUP_ADMIN), "Allows users to delete any other user")
-      ->addRow("Permission/fetch", array(USER_GROUP_ADMIN), "Allows users to list all API permissions")
-      ->addRow("Visitors/stats", array(USER_GROUP_ADMIN, USER_GROUP_SUPPORT), "Allows users to see visitor statistics")
-      ->addRow("Contact/respond", array(USER_GROUP_ADMIN, USER_GROUP_SUPPORT), "Allows users to respond to contact requests")
-      ->addRow("Contact/fetch", array(USER_GROUP_ADMIN, USER_GROUP_SUPPORT), "Allows users to fetch all contact requests")
-      ->addRow("Contact/get", array(USER_GROUP_ADMIN, USER_GROUP_SUPPORT), "Allows users to see messages within a contact request");
+      ->addRow("Groups/fetch", array(Group::SUPPORT, Group::ADMIN), "Allows users to list all available groups")
+      ->addRow("Groups/create", array(Group::ADMIN), "Allows users to create a new groups")
+      ->addRow("Groups/delete", array(Group::ADMIN), "Allows users to delete a group")
+      ->addRow("Routes/fetch", array(Group::ADMIN), "Allows users to list all configured routes")
+      ->addRow("Routes/save", array(Group::ADMIN), "Allows users to create, delete and modify routes")
+      ->addRow("Mail/test", array(Group::SUPPORT, Group::ADMIN), "Allows users to send a test email to a given address")
+      ->addRow("Mail/Sync", array(Group::SUPPORT, Group::ADMIN), "Allows users to synchronize mails with the database")
+      ->addRow("Settings/get", array(Group::ADMIN), "Allows users to fetch server settings")
+      ->addRow("Settings/set", array(Group::ADMIN), "Allows users create, delete or modify server settings")
+      ->addRow("Stats", array(Group::ADMIN, Group::SUPPORT), "Allows users to fetch server stats")
+      ->addRow("User/create", array(Group::ADMIN), "Allows users to create a new user, email address does not need to be confirmed")
+      ->addRow("User/fetch", array(Group::ADMIN, Group::SUPPORT), "Allows users to list all registered users")
+      ->addRow("User/get", array(Group::ADMIN, Group::SUPPORT), "Allows users to get information about a single user")
+      ->addRow("User/invite", array(Group::ADMIN), "Allows users to create a new user and send them an invitation link")
+      ->addRow("User/edit", array(Group::ADMIN), "Allows users to edit details and group memberships of any user")
+      ->addRow("User/delete", array(Group::ADMIN), "Allows users to delete any other user")
+      ->addRow("Permission/fetch", array(Group::ADMIN), "Allows users to list all API permissions")
+      ->addRow("Visitors/stats", array(Group::ADMIN, Group::SUPPORT), "Allows users to see visitor statistics")
+      ->addRow("Contact/respond", array(Group::ADMIN, Group::SUPPORT), "Allows users to respond to contact requests")
+      ->addRow("Contact/fetch", array(Group::ADMIN, Group::SUPPORT), "Allows users to fetch all contact requests")
+      ->addRow("Contact/get", array(Group::ADMIN, Group::SUPPORT), "Allows users to see messages within a contact request")
+      ->addRow("Logs/get", [Group::ADMIN], "Allows users to fetch system logs");
 
     self::loadPatches($queries, $sql);
 
@@ -195,9 +125,10 @@ class CreateDatabase extends DatabaseScript {
   }
 
   public static function loadEntities(&$queries, $sql) {
-    $handlers = [];
+    $persistables = [];
     $baseDirs = ["Core", "Site"];
     foreach ($baseDirs as $baseDir) {
+
       $entityDirectory = "./$baseDir/Objects/DatabaseEntity/";
       if (file_exists($entityDirectory) && is_dir($entityDirectory)) {
         $scan_arr = scandir($entityDirectory);
@@ -206,38 +137,40 @@ class CreateDatabase extends DatabaseScript {
           $suffix = ".class.php";
           if (endsWith($file, $suffix)) {
             $className = substr($file, 0, strlen($file) - strlen($suffix));
-            if (!in_array($className, ["DatabaseEntity", "DatabaseEntityQuery", "DatabaseEntityHandler"])) {
-              $className = "\\$baseDir\\Objects\\DatabaseEntity\\$className";
-              $reflectionClass = new \ReflectionClass($className);
-              if ($reflectionClass->isSubclassOf(DatabaseEntity::class)) {
-                $method = "$className::getHandler";
-                $handler = call_user_func($method, $sql);
-                $handlers[$handler->getTableName()] = $handler;
+            $className = "\\$baseDir\\Objects\\DatabaseEntity\\$className";
+            $reflectionClass = new \ReflectionClass($className);
+            if ($reflectionClass->isSubclassOf(DatabaseEntity::class)) {
+              $method = "$className::getHandler";
+              $handler = call_user_func($method, $sql);
+              $persistables[$handler->getTableName()] = $handler;
+
+              foreach ($handler->getNMRelations() as $nmTableName => $nmRelation) {
+                $persistables[$nmTableName] = $nmRelation;
               }
             }
           }
         }
       }
 
-      $tableCount = count($handlers);
+      $tableCount = count($persistables);
       $createdTables = [];
-      while (!empty($handlers)) {
+      while (!empty($persistables)) {
         $prevCount = $tableCount;
         $unmetDependenciesTotal = [];
 
-        foreach ($handlers as $tableName => $handler) {
-          $dependsOn = $handler->dependsOn();
+        foreach ($persistables as $tableName => $persistable) {
+          $dependsOn = $persistable->dependsOn();
           $unmetDependencies = array_diff($dependsOn, $createdTables);
           if (empty($unmetDependencies)) {
-            $queries[] = $handler->getTableQuery();
+            $queries = array_merge($queries, $persistable->getCreateQueries($sql));
             $createdTables[] = $tableName;
-            unset($handlers[$tableName]);
+            unset($persistables[$tableName]);
           } else {
             $unmetDependenciesTotal = array_merge($unmetDependenciesTotal, $unmetDependencies);
           }
         }
 
-        $tableCount = count($handlers);
+        $tableCount = count($persistables);
         if ($tableCount === $prevCount) {
           throw new Exception("Circular or unmet table dependency detected. Unmet dependencies: "
             . implode(", ", $unmetDependenciesTotal));
diff --git a/Core/Configuration/Patch/EntityLog_2021_04_08.class.php b/Core/Configuration/Patch/EntityLog_2021_04_08.class.php
index 1c23d4d..61f8c4c 100644
--- a/Core/Configuration/Patch/EntityLog_2021_04_08.class.php
+++ b/Core/Configuration/Patch/EntityLog_2021_04_08.class.php
@@ -5,7 +5,6 @@ namespace Core\Configuration\Patch;
 use Core\Configuration\DatabaseScript;
 use Core\Driver\SQL\Column\IntColumn;
 use Core\Driver\SQL\Condition\Compare;
-use Core\Driver\SQL\Query\CreateProcedure;
 use Core\Driver\SQL\SQL;
 use Core\Driver\SQL\Type\CurrentColumn;
 use Core\Driver\SQL\Type\CurrentTable;
@@ -13,32 +12,6 @@ use Core\Driver\SQL\Type\Trigger;
 
 class EntityLog_2021_04_08 extends DatabaseScript {
 
-  public static function createTableLog(SQL $sql, string $table, int $lifetime = 90): array {
-    return [
-      $sql->createTrigger("${table}_trg_insert")
-        ->after()->insert($table)
-        ->exec(new CreateProcedure($sql, "InsertEntityLog"), [
-          "tableName" => new CurrentTable(),
-          "entityId" => new CurrentColumn("id"),
-          "lifetime" => $lifetime,
-        ]),
-
-      $sql->createTrigger("${table}_trg_update")
-        ->after()->update($table)
-        ->exec(new CreateProcedure($sql, "UpdateEntityLog"), [
-          "tableName" => new CurrentTable(),
-          "entityId" => new CurrentColumn("id"),
-        ]),
-
-      $sql->createTrigger("${table}_trg_delete")
-        ->after()->delete($table)
-        ->exec(new CreateProcedure($sql, "DeleteEntityLog"), [
-          "tableName" => new CurrentTable(),
-          "entityId" => new CurrentColumn("id"),
-        ])
-    ];
-  }
-
   public static function createQueries(SQL $sql): array {
 
     $queries = array();
@@ -84,11 +57,6 @@ class EntityLog_2021_04_08 extends DatabaseScript {
     $queries[] = $updateProcedure;
     $queries[] = $deleteProcedure;
 
-    $tables = ["ContactRequest"];
-    foreach ($tables as $table) {
-      $queries = array_merge($queries, self::createTableLog($sql, $table));
-    }
-
     return $queries;
   }
 
diff --git a/Core/Configuration/Patch/SystemLog_2022_03_30.class.php b/Core/Configuration/Patch/SystemLog_2022_03_30.class.php
deleted file mode 100644
index ecc7ec9..0000000
--- a/Core/Configuration/Patch/SystemLog_2022_03_30.class.php
+++ /dev/null
@@ -1,16 +0,0 @@
-<?php
-
-namespace Core\Configuration\Patch;
-
-use Core\Configuration\DatabaseScript;
-use Core\Driver\SQL\SQL;
-
-class SystemLog_2022_03_30 extends DatabaseScript {
-
-  public static function createQueries(SQL $sql): array {
-    return [
-      $sql->insert("ApiPermission", ["method", "groups", "description"])
-        ->addRow("Logs/get", [USER_GROUP_ADMIN], "Allows users to fetch system logs")
-    ];
-  }
-}
\ No newline at end of file
diff --git a/Core/Configuration/Settings.class.php b/Core/Configuration/Settings.class.php
index 7d42e53..675b441 100644
--- a/Core/Configuration/Settings.class.php
+++ b/Core/Configuration/Settings.class.php
@@ -63,7 +63,7 @@ class Settings {
   }
 
   public static function loadDefaults(): Settings {
-    $hostname = $_SERVER["SERVER_NAME"];
+    $hostname = $_SERVER["SERVER_NAME"] ?? null;
     if (empty($hostname)) {
       $hostname = "localhost";
     }
@@ -190,7 +190,14 @@ class Settings {
       ->addRow("recaptcha_enabled", $this->recaptchaEnabled ? "1" : "0", false, false)
       ->addRow("recaptcha_public_key", $this->recaptchaPublicKey, false, false)
       ->addRow("recaptcha_private_key", $this->recaptchaPrivateKey, true, false)
-      ->addRow("allowed_extensions", implode(",", $this->allowedExtensions), true, false);
+      ->addRow("allowed_extensions", implode(",", $this->allowedExtensions), true, false)
+      ->addRow("mail_host", "", false, false)
+      ->addRow("mail_port", "", false, false)
+      ->addRow("mail_username", "", false, false)
+      ->addRow("mail_password", "", true, false)
+      ->addRow("mail_from", "", false, false)
+      ->addRow("mail_last_sync", "", false, false)
+      ->addRow("mail_footer", "", false, false);
   }
 
   public function getSiteName(): string {
diff --git a/Core/Documents/Info.class.php b/Core/Documents/Info.class.php
index aeef703..ec3fcd3 100644
--- a/Core/Documents/Info.class.php
+++ b/Core/Documents/Info.class.php
@@ -17,7 +17,7 @@ class Info extends HtmlDocument {
 class InfoBody extends SimpleBody {
   protected function getContent(): string {
     $user = $this->getDocument()->getUser();
-    if ($user && $user->hasGroup(USER_GROUP_ADMIN)) {
+    if ($user && $user->hasGroup(Group::ADMIN)) {
       phpinfo();
       return "";
     } else {
diff --git a/Core/Documents/Install.class.php b/Core/Documents/Install.class.php
index 47925c9..afd1721 100644
--- a/Core/Documents/Install.class.php
+++ b/Core/Documents/Install.class.php
@@ -30,6 +30,7 @@ namespace Documents\Install {
   use Core\External\PHPMailer\Exception;
   use Core\External\PHPMailer\PHPMailer;
   use Core\Objects\ConnectionData;
+  use Core\Objects\DatabaseEntity\Group;
 
   class InstallHead extends Head {
 
@@ -202,7 +203,7 @@ namespace Documents\Install {
             $req->execute(array(
                 "title" => "Welcome",
                 "message" => "Your Web-base was successfully installed. Check out the admin dashboard. Have fun!",
-                "groupId" => USER_GROUP_ADMIN
+                "groupId" => Group::ADMIN
               )
             );
             $this->errorString = $req->getLastError();
@@ -436,16 +437,10 @@ namespace Documents\Install {
           'email' => $email,
           'password' => $password,
           'confirmPassword' => $confirmPassword,
+          'groups' => [Group::ADMIN]
         ));
 
         $msg = $req->getLastError();
-        if ($success) {
-          $sql = $context->getSQL();
-          $success = $sql->insert("UserGroup", array("group_id", "user_id"))
-            ->addRow(USER_GROUP_ADMIN, $req->getResult()["userId"])
-            ->execute();
-          $msg = $sql->getLastError();
-        }
       }
 
       return array("msg" => $msg, "success" => $success);
diff --git a/Core/Driver/SQL/Query/Select.class.php b/Core/Driver/SQL/Query/Select.class.php
index 942b1d6..a77f0ac 100644
--- a/Core/Driver/SQL/Query/Select.class.php
+++ b/Core/Driver/SQL/Query/Select.class.php
@@ -38,6 +38,11 @@ class Select extends Query {
     $this->fetchType = SQL::FETCH_ALL;
   }
 
+  public function addColumn(string $columnName): Select {
+    $this->selectValues[] = $columnName;
+    return $this;
+  }
+
   public function from(...$tables): Select {
     $this->tables = array_merge($this->tables, $tables);
     return $this;
diff --git a/Core/Objects/Context.class.php b/Core/Objects/Context.class.php
index a3c415d..d227106 100644
--- a/Core/Objects/Context.class.php
+++ b/Core/Objects/Context.class.php
@@ -90,7 +90,7 @@ class Context {
     session_write_close();
   }
 
-  private function loadSession(int $userId, int $sessionId) {
+  private function loadSession(int $userId, int $sessionId): void {
     $this->session = Session::init($this, $userId, $sessionId);
     $this->user = $this->session?->getUser();
     if ($this->user) {
@@ -128,12 +128,13 @@ class Context {
 
   public function updateLanguage(string $lang): bool {
     if ($this->sql) {
-      $language = Language::findBuilder($this->sql)
+      $language = Language::findBy(Language::createBuilder($this->sql, true)
         ->where(new CondOr(
             new CondLike("name", "%$lang%"), // english
             new Compare("code", $lang), // de_DE
-            new CondLike("code", $lang . "_%"))) // de -> de_%
-        ->execute();
+            new CondLike("code", "${lang}_%") // de -> de_%
+        ))
+      );
       if ($language) {
         $this->setLanguage($language);
         return true;
@@ -176,14 +177,13 @@ class Context {
   }
 
   public function loadApiKey(string $apiKey): bool {
-    $this->user = User::findBuilder($this->sql)
+    $this->user = User::findBy(User::createBuilder($this->sql, true)
       ->addJoin(new Join("INNER","ApiKey", "ApiKey.user_id", "User.id"))
       ->where(new Compare("ApiKey.api_key", $apiKey))
       ->where(new Compare("valid_until", $this->sql->currentTimestamp(), ">"))
       ->where(new Compare("ApiKey.active", true))
       ->where(new Compare("User.confirmed", true))
-      ->fetchEntities()
-      ->execute();
+      ->fetchEntities());
 
     return $this->user !== null;
   }
diff --git a/Core/Objects/DatabaseEntity/ApiKey.class.php b/Core/Objects/DatabaseEntity/ApiKey.class.php
index 7a99ff6..d9e0719 100644
--- a/Core/Objects/DatabaseEntity/ApiKey.class.php
+++ b/Core/Objects/DatabaseEntity/ApiKey.class.php
@@ -3,6 +3,7 @@
 namespace Core\Objects\DatabaseEntity;
 
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
 class ApiKey extends DatabaseEntity {
 
diff --git a/Core/Objects/DatabaseEntity/Attribute/Multiple.php b/Core/Objects/DatabaseEntity/Attribute/Multiple.php
new file mode 100644
index 0000000..ec62963
--- /dev/null
+++ b/Core/Objects/DatabaseEntity/Attribute/Multiple.php
@@ -0,0 +1,18 @@
+<?php
+
+namespace Core\Objects\DatabaseEntity\Attribute;
+
+#[\Attribute(\Attribute::TARGET_PROPERTY)]
+class Multiple {
+
+  private string $className;
+
+  public function __construct(string $className) {
+    $this->className = $className;
+  }
+
+  public function getClassName(): string {
+    return $this->className;
+  }
+
+}
\ No newline at end of file
diff --git a/Core/Objects/DatabaseEntity/DatabaseEntity.class.php b/Core/Objects/DatabaseEntity/Controller/DatabaseEntity.class.php
similarity index 73%
rename from Core/Objects/DatabaseEntity/DatabaseEntity.class.php
rename to Core/Objects/DatabaseEntity/Controller/DatabaseEntity.class.php
index 58a406f..3814efb 100644
--- a/Core/Objects/DatabaseEntity/DatabaseEntity.class.php
+++ b/Core/Objects/DatabaseEntity/Controller/DatabaseEntity.class.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Core\Objects\DatabaseEntity;
+namespace Core\Objects\DatabaseEntity\Controller;
 
 use Core\Driver\SQL\Condition\Compare;
 use Core\Driver\SQL\Condition\Condition;
@@ -8,6 +8,13 @@ use Core\Driver\SQL\SQL;
 
 abstract class DatabaseEntity {
 
+  protected static array $entityLogConfig = [
+    "insert" => false,
+    "update" => false,
+    "delete" => false,
+    "lifetime" => null,
+  ];
+
   private static array $handlers = [];
   protected ?int $id;
 
@@ -51,8 +58,8 @@ abstract class DatabaseEntity {
     return $res !== false && $res[0]["count"] !== 0;
   }
 
-  public static function findBuilder(SQL $sql): DatabaseEntityQuery {
-    return DatabaseEntityQuery::fetchOne(self::getHandler($sql));
+  public static function findBy(DatabaseEntityQuery $dbQuery): static|array|bool|null {
+    return $dbQuery->execute();
   }
 
   public static function findAll(SQL $sql, ?Condition $condition = null): ?array {
@@ -60,17 +67,22 @@ abstract class DatabaseEntity {
     return $handler->fetchMultiple($condition);
   }
 
-  public static function findAllBuilder(SQL $sql): DatabaseEntityQuery {
-    return DatabaseEntityQuery::fetchAll(self::getHandler($sql));
+  public static function createBuilder(SQL $sql, bool $one): DatabaseEntityQuery {
+    if ($one) {
+      return DatabaseEntityQuery::fetchOne(self::getHandler($sql));
+    } else {
+      return DatabaseEntityQuery::fetchAll(self::getHandler($sql));
+    }
   }
 
-  public function save(SQL $sql, ?array $columns = null): bool {
+  public function save(SQL $sql, ?array $columns = null, bool $saveNM = false): bool {
     $handler = self::getHandler($sql);
-    $res = $handler->insertOrUpdate($this, $columns);
+    $res = $handler->insertOrUpdate($this, $columns, $saveNM);
     if ($res === false) {
       return false;
     } else if ($this->id === null) {
       $this->id = $res;
+      $handler->insertNM($this);
     }
 
     return true;
@@ -127,4 +139,22 @@ abstract class DatabaseEntity {
   public function getId(): ?int {
     return $this->id;
   }
+
+  public static function count(SQL $sql, ?Condition $condition = null): int|bool {
+    $handler = self::getHandler($sql);
+    $query = $sql->select($sql->count())
+      ->from($handler->getTableName());
+
+    if ($condition) {
+      $query->where($condition);
+    }
+
+    $res = $query->execute();
+
+    if (!empty($res)) {
+      return $res[0]["count"];
+    }
+
+    return false;
+  }
 }
\ No newline at end of file
diff --git a/Core/Objects/DatabaseEntity/DatabaseEntityHandler.php b/Core/Objects/DatabaseEntity/Controller/DatabaseEntityHandler.php
similarity index 51%
rename from Core/Objects/DatabaseEntity/DatabaseEntityHandler.php
rename to Core/Objects/DatabaseEntity/Controller/DatabaseEntityHandler.php
index 6c201c1..180afca 100644
--- a/Core/Objects/DatabaseEntity/DatabaseEntityHandler.php
+++ b/Core/Objects/DatabaseEntity/Controller/DatabaseEntityHandler.php
@@ -1,33 +1,46 @@
 <?php
 
-namespace Core\Objects\DatabaseEntity;
+namespace Core\Objects\DatabaseEntity\Controller;
 
 use Core\Driver\Logger\Logger;
 use Core\Driver\SQL\Column\BoolColumn;
+use Core\Driver\SQL\Column\Column;
 use Core\Driver\SQL\Column\DateTimeColumn;
 use Core\Driver\SQL\Column\EnumColumn;
 use Core\Driver\SQL\Column\IntColumn;
 use Core\Driver\SQL\Column\JsonColumn;
 use Core\Driver\SQL\Column\StringColumn;
 use Core\Driver\SQL\Condition\Compare;
+use Core\Driver\SQL\Condition\CondAnd;
+use Core\Driver\SQL\Condition\CondBool;
+use Core\Driver\SQL\Condition\CondIn;
 use Core\Driver\SQL\Condition\Condition;
 use Core\Driver\SQL\Column\DoubleColumn;
 use Core\Driver\SQL\Column\FloatColumn;
+use Core\Driver\SQL\Condition\CondNot;
+use Core\Driver\SQL\Condition\CondOr;
 use Core\Driver\SQL\Constraint\ForeignKey;
+use Core\Driver\SQL\Join;
+use Core\Driver\SQL\Query\CreateProcedure;
 use Core\Driver\SQL\Query\CreateTable;
+use Core\Driver\SQL\Query\Insert;
 use Core\Driver\SQL\Query\Select;
 use Core\Driver\SQL\SQL;
 use Core\Driver\SQL\Strategy\CascadeStrategy;
 use Core\Driver\SQL\Strategy\SetNullStrategy;
+use Core\Driver\SQL\Strategy\UpdateStrategy;
+use Core\Driver\SQL\Type\CurrentColumn;
+use Core\Driver\SQL\Type\CurrentTable;
 use Core\Objects\DatabaseEntity\Attribute\Enum;
 use Core\Objects\DatabaseEntity\Attribute\DefaultValue;
 use Core\Objects\DatabaseEntity\Attribute\Json;
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
+use Core\Objects\DatabaseEntity\Attribute\Multiple;
 use Core\Objects\DatabaseEntity\Attribute\Transient;
 use Core\Objects\DatabaseEntity\Attribute\Unique;
 use PHPUnit\Util\Exception;
 
-class DatabaseEntityHandler {
+class DatabaseEntityHandler implements Persistable {
 
   private \ReflectionClass $entityClass;
   private string $tableName;
@@ -35,6 +48,7 @@ class DatabaseEntityHandler {
   private array $properties;
   private array $relations;
   private array $constraints;
+  private array $nmRelations;
   private SQL $sql;
   private Logger $logger;
 
@@ -50,8 +64,9 @@ class DatabaseEntityHandler {
     $this->tableName = $this->entityClass->getShortName();
     $this->columns = [];     // property name => database column name
     $this->properties = [];  // property name => \ReflectionProperty
-    $this->relations = [];   // property name => referenced table name
+    $this->relations = [];   // property name => DatabaseEntityHandler
     $this->constraints = []; // \Driver\SQL\Constraint\Constraint
+    $this->nmRelations = [];    // table name => NMRelation
 
     foreach ($this->entityClass->getProperties() as $property) {
       $propertyName = $property->getName();
@@ -60,6 +75,10 @@ class DatabaseEntityHandler {
         continue;
       }
 
+      if ($property->isStatic()) {
+        continue;
+      }
+
       $propertyType = $property->getType();
       $columnName = self::getColumnName($propertyName);
       if (!($propertyType instanceof \ReflectionNamedType)) {
@@ -93,18 +112,46 @@ class DatabaseEntityHandler {
         $this->columns[$propertyName] = new BoolColumn($columnName, $defaultValue ?? false);
       } else if ($propertyTypeName === 'DateTime') {
         $this->columns[$propertyName] = new DateTimeColumn($columnName, $nullable, $defaultValue);
-      /*} else if ($propertyName === 'array') {
-        $many = self::getAttribute($property, Many::class);
-        if ($many) {
-          $requestedType = $many->getValue();
-          if (isClass($requestedType)) {
-            $requestedClass = new \ReflectionClass($requestedType);
+        /*} else if ($propertyName === 'array') {
+          $many = self::getAttribute($property, Many::class);
+          if ($many) {
+            $requestedType = $many->getValue();
+            if (isClass($requestedType)) {
+              $requestedClass = new \ReflectionClass($requestedType);
+            } else {
+              $this->raiseError("Cannot persist class '$className': Property '$propertyName' has non persist-able type: $requestedType");
+            }
           } else {
-            $this->raiseError("Cannot persist class '$className': Property '$propertyName' has non persist-able type: $requestedType");
+            $this->raiseError("Cannot persist class '$className': Property '$propertyName' has non persist-able type: $propertyTypeName");
+          }*/
+      } else if ($propertyTypeName === "array") {
+        $multiple = self::getAttribute($property, Multiple::class);
+        if (!$multiple) {
+          $this->raiseError("Cannot persist class '$className': Property '$propertyName' has non persist-able type: $propertyTypeName. " .
+                            "Is the 'Multiple' attribute missing?");
+        }
+
+        try {
+          $refClass = $multiple->getClassName();
+          $requestedClass = new \ReflectionClass($refClass);
+          if ($requestedClass->isSubclassOf(DatabaseEntity::class)) {
+            $nmTableName = NMRelation::buildTableName($this->getTableName(), $requestedClass->getShortName());
+            $nmRelation = $this->nmRelations[$nmTableName] ?? null;
+            if (!$nmRelation) {
+              $otherHandler = DatabaseEntity::getHandler($this->sql, $requestedClass);
+              $otherNM = $otherHandler->getNMRelations();
+              $nmRelation = $otherNM[$nmTableName] ?? (new NMRelation($this, $otherHandler));
+              $this->nmRelations[$nmTableName] = $nmRelation;
+            }
+
+            $this->nmRelations[$nmTableName]->addProperty($this, $property);
+          } else {
+            $this->raiseError("Cannot persist class '$className': Property '$propertyName' of type multiple can " .
+                              "only reference DatabaseEntity types, but got: $refClass");
           }
-        } else {
-          $this->raiseError("Cannot persist class '$className': Property '$propertyName' has non persist-able type: $propertyTypeName");
-        }*/
+        } catch (\Exception $ex) {
+          $this->raiseError("Cannot persist class '$className' property '$propertyTypeName': " . $ex->getMessage());
+        }
       } else if ($propertyTypeName !== "mixed") {
         try {
           $requestedClass = new \ReflectionClass($propertyTypeName);
@@ -167,6 +214,10 @@ class DatabaseEntityHandler {
     return $this->relations;
   }
 
+  public function getNMRelations(): array {
+    return $this->nmRelations;
+  }
+
   public function getColumnNames(): array {
     $columns = ["$this->tableName.id"];
     foreach ($this->columns as $column) {
@@ -238,6 +289,14 @@ class DatabaseEntityHandler {
         }
       }
 
+      // init n:m / 1:n properties with empty arrays
+      foreach ($this->nmRelations as $nmRelation) {
+        foreach ($nmRelation->getProperties($this) as $property) {
+          $property->setAccessible(true);
+          $property->setValue($entity, []);
+        }
+      }
+
       $this->properties["id"]->setAccessible(true);
       $this->properties["id"]->setValue($entity, $row["id"]);
       $entity->postFetch($this->sql, $row);
@@ -248,6 +307,190 @@ class DatabaseEntityHandler {
     }
   }
 
+  public function updateNM(DatabaseEntity $entity): bool {
+    if (empty($this->nmRelations)) {
+      return true;
+    }
+
+    foreach ($this->nmRelations as $nmTable => $nmRelation) {
+
+      $thisIdColumn = $nmRelation->getIdColumn($this);
+      $thisTableName = $this->getTableName();
+      $dataColumns = $nmRelation->getDataColumns();
+      $otherHandler = $nmRelation->getOtherHandler($this);
+      $refIdColumn = $nmRelation->getIdColumn($otherHandler);
+
+
+      // delete from n:m table if no longer exists
+      $deleteStatement = $this->sql->delete($nmTable)
+        ->where(new Compare($thisIdColumn, $entity->getId()));
+
+      if (!empty($dataColumns)) {
+        $conditions = [];
+        foreach ($dataColumns[$thisTableName] as $propertyName => $columnName) {
+          $property = $this->properties[$propertyName];
+          $entityIds = array_keys($property->getValue($entity));
+          if (!empty($entityIds)) {
+            $conditions[] = new CondAnd(
+              new CondBool($columnName),
+              new CondNot(new CondIn(new Column($refIdColumn), $entityIds)),
+            );
+          }
+        }
+
+        if (!empty($conditions)) {
+          $deleteStatement->where(new CondOr(...$conditions));
+        }
+      } else {
+        $property = next($nmRelation->getProperties($this));
+        $entityIds = array_keys($property->getValue($entity));
+        if (!empty($entityIds)) {
+          $deleteStatement->where(
+            new CondNot(new CondIn(new Column($refIdColumn), $entityIds))
+          );
+        }
+      }
+
+      $deleteStatement->execute();
+    }
+
+    return $this->insertNM($entity, true);
+  }
+
+  public function insertNM(DatabaseEntity $entity, bool $ignoreExisting = true): bool {
+
+    if (empty($this->nmRelations)) {
+      return true;
+    }
+
+    $success = true;
+    foreach ($this->nmRelations as $nmTable => $nmRelation) {
+      $otherHandler = $nmRelation->getOtherHandler($this);
+      $thisIdColumn = $nmRelation->getIdColumn($this);
+      $thisTableName = $this->getTableName();
+      $refIdColumn = $nmRelation->getIdColumn($otherHandler);
+      $dataColumns = $nmRelation->getDataColumns();
+
+      $columns = [
+        $thisIdColumn,
+        $refIdColumn,
+      ];
+
+      if (!empty($dataColumns)) {
+        $columns = array_merge($columns, array_values($dataColumns[$thisTableName]));
+      }
+
+      $statement = $this->sql->insert($nmTable, $columns);
+      if ($ignoreExisting) {
+        $statement->onDuplicateKeyStrategy(new UpdateStrategy($nmRelation->getAllColumns(), [
+          $thisIdColumn => $entity->getId()
+        ]));
+      }
+
+      foreach ($nmRelation->getProperties($this) as $property) {
+        $property->setAccessible(true);
+        $relEntities = $property->getValue($entity);
+        foreach ($relEntities as $relEntity) {
+          $nmRow = [$entity->getId(), $relEntity->getId()];
+          if (!empty($dataColumns)) {
+            foreach (array_keys($dataColumns[$thisTableName]) as $propertyName) {
+              $nmRow[] = $property->getName() === $propertyName;
+            }
+          }
+          $statement->addRow(...$nmRow);
+        }
+      }
+
+      $success = $statement->execute() && $success;
+    }
+
+    return $success;
+  }
+
+  public function fetchNMRelations(array $entities, bool $recursive = false) {
+
+    if ($recursive) {
+      foreach ($entities as $entity) {
+        foreach ($this->relations as $propertyName => $relHandler) {
+          $relEntity = $this->properties[$propertyName]->getValue($entity);
+          if ($relEntity) {
+            $relHandler->fetchNMRelations([$relEntity->getId() => $relEntity], true);
+          }
+        }
+      }
+    }
+
+    if (empty($this->nmRelations)) {
+      return;
+    }
+
+    $entityIds = array_keys($entities);
+    foreach ($this->nmRelations as $nmTable => $nmRelation) {
+      $otherHandler = $nmRelation->getOtherHandler($this);
+
+      $thisIdColumn = $nmRelation->getIdColumn($this);
+      $thisProperties = $nmRelation->getProperties($this);
+      $thisTableName = $this->getTableName();
+
+      $refIdColumn = $nmRelation->getIdColumn($otherHandler);
+      $refProperties = $nmRelation->getProperties($otherHandler);
+      $refTableName = $otherHandler->getTableName();
+
+      $dataColumns = $nmRelation->getDataColumns();
+
+      $relEntityQuery = DatabaseEntityQuery::fetchAll($otherHandler)
+        ->addJoin(new Join("INNER", $nmTable, "$nmTable.$refIdColumn", "$refTableName.id"))
+        ->where(new CondIn(new Column($thisIdColumn), $entityIds))
+        ->getQuery();
+
+      $relEntityQuery->addColumn($thisIdColumn);
+      foreach ($dataColumns as $tableDataColumns) {
+        foreach ($tableDataColumns as $columnName) {
+          $relEntityQuery->addColumn($columnName);
+        }
+      }
+
+      $rows = $relEntityQuery->execute();
+      if (!is_array($rows)) {
+        $this->logger->error("Error fetching n:m relations from table: '$nmTable': " . $this->sql->getLastError());
+        return;
+      }
+
+      $relEntities = [];
+      foreach ($rows as $row) {
+        $relId = $row["id"];
+        if (!isset($relEntities[$relId])) {
+          $relEntity = $otherHandler->entityFromRow($row);
+          $relEntities[$relId] = $relEntity;
+        }
+
+        $thisEntity = $entities[$row[$thisIdColumn]];
+        $relEntity = $relEntities[$relId];
+        $mappings = [
+          [$refProperties, $refTableName, $relEntity, $thisEntity],
+          [$thisProperties, $thisTableName, $thisEntity, $relEntity],
+        ];
+
+        foreach ($mappings as $mapping) {
+          list($properties, $tableName, $targetEntity, $entityToAdd) = $mapping;
+          foreach ($properties as $propertyName => $property) {
+            $addToProperty = empty($dataColumns);
+            if (!$addToProperty) {
+              $columnName = $dataColumns[$tableName][$propertyName] ?? null;
+              $addToProperty = ($columnName && $this->sql->parseBool($row[$columnName]));
+            }
+
+            if ($addToProperty) {
+              $targetArray = $property->getValue($targetEntity);
+              $targetArray[$entityToAdd->getId()] = $entityToAdd;
+              $property->setValue($targetEntity, $targetArray);
+            }
+          }
+        }
+      }
+    }
+  }
+
   public function getSelectQuery(): Select {
     return $this->sql->select(...$this->getColumnNames())
       ->from($this->tableName);
@@ -259,11 +502,14 @@ class DatabaseEntityHandler {
       ->first()
       ->execute();
 
-    if ($res === false || $res === null) {
-      return $res;
-    } else {
-      return $this->entityFromRow($res);
+    if ($res !== false && $res !== null) {
+      $res = $this->entityFromRow($res);
+      if ($res instanceof DatabaseEntity) {
+        $this->fetchNMRelations([$res->getId() => $res]);
+      }
     }
+
+    return $res;
   }
 
   public function fetchMultiple(?Condition $condition = null): ?array {
@@ -284,12 +530,56 @@ class DatabaseEntityHandler {
           $entities[$entity->getId()] = $entity;
         }
       }
+
+      $this->fetchNMRelations($entities);
       return $entities;
     }
   }
 
-  public function getTableQuery(): CreateTable {
-    $query = $this->sql->createTable($this->tableName)
+  public function getCreateQueries(SQL $sql): array {
+
+    $queries = [];
+    $queries[] = $this->getTableQuery($sql);
+
+    $table = $this->getTableName();
+    $entityLogConfig = $this->entityClass->getProperty("entityLogConfig");
+    $entityLogConfig->setAccessible(true);
+    $entityLogConfig = $entityLogConfig->getValue();
+
+    if (isset($entityLogConfig["insert"]) && $entityLogConfig["insert"] === true) {
+      $queries[] = $sql->createTrigger("${table}_trg_insert")
+        ->after()->insert($table)
+        ->exec(new CreateProcedure($sql, "InsertEntityLog"), [
+          "tableName" => new CurrentTable(),
+          "entityId" => new CurrentColumn("id"),
+          "lifetime" => $entityLogConfig["lifetime"] ?? 90,
+        ]);
+    }
+
+    if (isset($entityLogConfig["update"]) && $entityLogConfig["update"] === true) {
+      $queries[] = $sql->createTrigger("${table}_trg_update")
+        ->after()->update($table)
+        ->exec(new CreateProcedure($sql, "UpdateEntityLog"), [
+          "tableName" => new CurrentTable(),
+          "entityId" => new CurrentColumn("id"),
+        ]);
+    }
+
+    if (isset($entityLogConfig["delete"]) && $entityLogConfig["delete"] === true) {
+      $queries[] = $sql->createTrigger("${table}_trg_delete")
+        ->after()->delete($table)
+        ->exec(new CreateProcedure($sql, "DeleteEntityLog"), [
+          "tableName" => new CurrentTable(),
+          "entityId" => new CurrentColumn("id"),
+        ]);
+    }
+
+
+    return $queries;
+  }
+
+  public function getTableQuery(SQL $sql): CreateTable {
+    $query = $sql->createTable($this->tableName)
       ->onlyIfNotExists()
       ->addSerial("id")
       ->primaryKey("id");
@@ -305,12 +595,7 @@ class DatabaseEntityHandler {
     return $query;
   }
 
-  public function createTable(): bool {
-    $query = $this->getTableQuery();
-    return $query->execute();
-  }
-
-  private function prepareRow(DatabaseEntity $entity, string $action, ?array $columns = null) {
+  private function prepareRow(DatabaseEntity $entity, string $action, ?array $columns = null): bool|array {
     $row = [];
     foreach ($this->columns as $propertyName => $column) {
       if ($columns && !in_array($column->getName(), $columns)) {
@@ -344,7 +629,7 @@ class DatabaseEntityHandler {
     return $row;
   }
 
-  public function update(DatabaseEntity $entity, ?array $columns = null) {
+  public function update(DatabaseEntity $entity, ?array $columns = null, bool $saveNM = false) {
     $row = $this->prepareRow($entity, "update", $columns);
     if ($row === false) {
       return false;
@@ -358,10 +643,15 @@ class DatabaseEntityHandler {
       $query->set($columnName, $value);
     }
 
-    return $query->execute();
+    $res = $query->execute();
+    if ($res && $saveNM) {
+      $res = $this->updateNM($entity);
+    }
+
+    return $res;
   }
 
-  public function insert(DatabaseEntity $entity) {
+  public function insert(DatabaseEntity $entity): bool|int {
     $row = $this->prepareRow($entity, "insert");
     if ($row === false) {
       return false;
@@ -391,12 +681,12 @@ class DatabaseEntityHandler {
     }
   }
 
-  public function insertOrUpdate(DatabaseEntity $entity, ?array $columns = null) {
+  public function insertOrUpdate(DatabaseEntity $entity, ?array $columns = null, bool $saveNM = false) {
     $id = $entity->getId();
     if ($id === null) {
       return $this->insert($entity);
     } else {
-      return $this->update($entity, $columns);
+      return $this->update($entity, $columns, $saveNM);
     }
   }
 
@@ -415,4 +705,30 @@ class DatabaseEntityHandler {
   public function getSQL(): SQL {
     return $this->sql;
   }
+
+  public function getInsertQuery(DatabaseEntity|array $entities): ?Insert {
+
+    if (empty($entities)) {
+      return null;
+    }
+
+    $firstEntity = (is_array($entities) ? current($entities) : $entities);
+    $firstRow = $this->prepareRow($firstEntity, "insert");
+
+    $statement = $this->sql->insert($this->tableName, array_keys($firstRow))
+      ->addRow(...array_values($firstRow));
+
+    if (is_array($entities)) {
+      foreach ($entities as $entity) {
+        if ($entity === $firstEntity) {
+          continue;
+        }
+
+        $row = $this->prepareRow($entity, "insert");
+        $statement->addRow(...array_values($row));
+      }
+    }
+
+    return $statement;
+  }
 }
\ No newline at end of file
diff --git a/Core/Objects/DatabaseEntity/DatabaseEntityQuery.class.php b/Core/Objects/DatabaseEntity/Controller/DatabaseEntityQuery.class.php
similarity index 83%
rename from Core/Objects/DatabaseEntity/DatabaseEntityQuery.class.php
rename to Core/Objects/DatabaseEntity/Controller/DatabaseEntityQuery.class.php
index 842372c..add000e 100644
--- a/Core/Objects/DatabaseEntity/DatabaseEntityQuery.class.php
+++ b/Core/Objects/DatabaseEntity/Controller/DatabaseEntityQuery.class.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Core\Objects\DatabaseEntity;
+namespace Core\Objects\DatabaseEntity\Controller;
 
 use Core\Driver\Logger\Logger;
 use Core\Driver\SQL\Condition\Condition;
@@ -14,18 +14,25 @@ use Core\Driver\SQL\SQL;
 */
 class DatabaseEntityQuery {
 
+  const FETCH_NONE = 0;
+  const FETCH_DIRECT = 1;
+  const FETCH_RECURSIVE = 2;
+
   private Logger $logger;
   private DatabaseEntityHandler $handler;
   private Select $selectQuery;
   private int $resultType;
   private bool $logVerbose;
 
+  private int $fetchSubEntities;
+
   private function __construct(DatabaseEntityHandler $handler, int $resultType) {
     $this->handler = $handler;
     $this->selectQuery = $handler->getSelectQuery();
     $this->logger = new Logger("DB-EntityQuery", $handler->getSQL());
     $this->resultType = $resultType;
     $this->logVerbose = false;
+    $this->fetchSubEntities = self::FETCH_NONE;
 
     if ($this->resultType === SQL::FETCH_ONE) {
       $this->selectQuery->first();
@@ -50,6 +57,11 @@ class DatabaseEntityQuery {
     return $this;
   }
 
+  public function offset(int $offset): static {
+    $this->selectQuery->offset($offset);
+    return $this;
+  }
+
   public function where(Condition ...$condition): DatabaseEntityQuery {
     $this->selectQuery->where(...$condition);
     return $this;
@@ -74,6 +86,7 @@ class DatabaseEntityQuery {
   public function fetchEntities(bool $recursive = false): DatabaseEntityQuery {
 
     // $this->selectQuery->dump();
+    $this->fetchSubEntities = ($recursive ? self::FETCH_RECURSIVE : self::FETCH_DIRECT);
 
     $relIndex = 1;
     foreach ($this->handler->getRelations() as $propertyName => $relationHandler) {
@@ -136,9 +149,19 @@ class DatabaseEntityQuery {
           $entities[$entity->getId()] = $entity;
         }
       }
+
+      if ($this->fetchSubEntities !== self::FETCH_NONE) {
+        $this->handler->fetchNMRelations($entities, $this->fetchSubEntities === self::FETCH_RECURSIVE);
+      }
+
       return $entities;
     } else if ($this->resultType === SQL::FETCH_ONE) {
-      return $this->handler->entityFromRow($res);
+      $entity = $this->handler->entityFromRow($res);
+      if ($entity instanceof DatabaseEntity && $this->fetchSubEntities !== self::FETCH_NONE) {
+        $this->handler->fetchNMRelations([$entity->getId() => $entity], $this->fetchSubEntities === self::FETCH_RECURSIVE);
+      }
+
+      return $entity;
     } else {
       $this->handler->getLogger()->error("Invalid result type for query builder, must be FETCH_ALL or FETCH_ONE");
       return null;
@@ -149,4 +172,8 @@ class DatabaseEntityQuery {
     $this->selectQuery->addJoin($join);
     return $this;
   }
+
+  public function getQuery(): Select {
+    return $this->selectQuery;
+  }
 }
\ No newline at end of file
diff --git a/Core/Objects/DatabaseEntity/Controller/NMRelation.class.php b/Core/Objects/DatabaseEntity/Controller/NMRelation.class.php
new file mode 100644
index 0000000..d85a188
--- /dev/null
+++ b/Core/Objects/DatabaseEntity/Controller/NMRelation.class.php
@@ -0,0 +1,134 @@
+<?php
+
+namespace Core\Objects\DatabaseEntity\Controller;
+
+# TODO: Allow more than 2 relations here?
+
+use Core\Driver\SQL\Query\CreateTable;
+use Core\Driver\SQL\SQL;
+use Core\Driver\SQL\Strategy\CascadeStrategy;
+
+class NMRelation implements Persistable {
+
+  private DatabaseEntityHandler $handlerA;
+  private DatabaseEntityHandler $handlerB;
+  private array $properties;
+
+  public function __construct(DatabaseEntityHandler $handlerA, DatabaseEntityHandler $handlerB) {
+    $this->handlerA = $handlerA;
+    $this->handlerB = $handlerB;
+    $tableNameA = $handlerA->getTableName();
+    $tableNameB = $handlerB->getTableName();
+    if ($tableNameA === $tableNameB) {
+      throw new \Exception("Cannot create N:M Relation with only one table");
+    }
+
+    $this->properties = [
+      $tableNameA => [],
+      $tableNameB => [],
+    ];
+  }
+
+  public function addProperty(DatabaseEntityHandler $src, \ReflectionProperty $property): void {
+    $this->properties[$src->getTableName()][$property->getName()] = $property;
+  }
+
+  public function getIdColumn(DatabaseEntityHandler $handler): string {
+    return DatabaseEntityHandler::getColumnName($handler->getTableName()) . "_id";
+  }
+
+  public function getDataColumns(): array {
+
+    $referenceCount = 0;
+    $columnsNeeded = false;
+
+    // if in one of the relations we have multiple references, we need to differentiate
+    foreach ($this->properties as $refProperties) {
+      $referenceCount += count($refProperties);
+      if ($referenceCount > 1) {
+        $columnsNeeded = true;
+        break;
+      }
+    }
+
+    $columns = [];
+    if ($columnsNeeded) {
+      foreach ($this->properties as $tableName => $properties) {
+        $columns[$tableName] = [];
+        foreach ($properties as $property) {
+          $columnName = DatabaseEntityHandler::getColumnName($tableName) . "_" .
+            DatabaseEntityHandler::getColumnName($property->getName());
+          $columns[$tableName][$property->getName()] = $columnName;
+        }
+      }
+    }
+
+    return $columns;
+  }
+
+  public function getAllColumns(): array {
+    $relIdA = $this->getIdColumn($this->handlerA);
+    $relIdB = $this->getIdColumn($this->handlerB);
+
+    $columns = [$relIdA, $relIdB];
+
+    foreach ($this->getDataColumns() as $dataColumns) {
+      foreach ($dataColumns as $columnName) {
+        $columns[] = $columnName;
+      }
+    }
+
+    return $columns;
+  }
+
+  public function getTableQuery(SQL $sql): CreateTable {
+
+    $tableNameA = $this->handlerA->getTableName();
+    $tableNameB = $this->handlerB->getTableName();
+
+    $columns = $this->getAllColumns();
+    list ($relIdA, $relIdB) = $columns;
+    $dataColumns = array_slice($columns, 2);
+    $query = $sql->createTable(self::buildTableName($tableNameA, $tableNameB))
+      ->addInt($relIdA)
+      ->addInt($relIdB)
+      ->foreignKey($relIdA, $tableNameA, "id", new CascadeStrategy())
+      ->foreignKey($relIdB, $tableNameB, "id", new CascadeStrategy());
+
+    foreach ($dataColumns as $dataColumn) {
+      $query->addBool($dataColumn, false);
+    }
+
+    $query->unique(...$columns);
+    return $query;
+  }
+
+  public static function buildTableName(string ...$tables): string {
+    sort($tables);
+    return "NM_" . implode("_", $tables);
+  }
+
+  public function dependsOn(): array {
+    return [$this->handlerA->getTableName(), $this->handlerB->getTableName()];
+  }
+
+  public function getTableName(): string {
+    return self::buildTableName(...$this->dependsOn());
+  }
+
+  public function getCreateQueries(SQL $sql): array {
+    return [$this->getTableQuery($sql)];
+  }
+
+  public function getProperties(DatabaseEntityHandler $handler): array {
+    return $this->properties[$handler->getTableName()];
+  }
+
+  public function getOtherHandler(DatabaseEntityHandler $handler): DatabaseEntityHandler {
+    if ($handler === $this->handlerA) {
+      return $this->handlerB;
+    } else {
+      return $this->handlerA;
+    }
+  }
+}
\ No newline at end of file
diff --git a/Core/Objects/DatabaseEntity/Controller/Persistable.interface.php b/Core/Objects/DatabaseEntity/Controller/Persistable.interface.php
new file mode 100644
index 0000000..39394f7
--- /dev/null
+++ b/Core/Objects/DatabaseEntity/Controller/Persistable.interface.php
@@ -0,0 +1,13 @@
+<?php
+
+namespace Core\Objects\DatabaseEntity\Controller;
+
+use Core\Driver\SQL\SQL;
+
+interface Persistable {
+
+  public function dependsOn(): array;
+  public function getTableName(): string;
+  public function getCreateQueries(SQL $sql): array;
+
+}
\ No newline at end of file
diff --git a/Core/Objects/DatabaseEntity/GpgKey.class.php b/Core/Objects/DatabaseEntity/GpgKey.class.php
index 6a41b85..1535ce1 100644
--- a/Core/Objects/DatabaseEntity/GpgKey.class.php
+++ b/Core/Objects/DatabaseEntity/GpgKey.class.php
@@ -6,6 +6,7 @@ use Core\Driver\SQL\Expression\CurrentTimeStamp;
 use Core\Driver\SQL\SQL;
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
 use Core\Objects\DatabaseEntity\Attribute\DefaultValue;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
 class GpgKey extends DatabaseEntity {
 
diff --git a/Core/Objects/DatabaseEntity/Group.class.php b/Core/Objects/DatabaseEntity/Group.class.php
index 529468b..bbbaa9b 100644
--- a/Core/Objects/DatabaseEntity/Group.class.php
+++ b/Core/Objects/DatabaseEntity/Group.class.php
@@ -3,14 +3,27 @@
 namespace Core\Objects\DatabaseEntity;
 
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
 class Group extends DatabaseEntity {
 
+  const ADMIN = 1;
+  const MODERATOR = 3;
+  const SUPPORT = 2;
+
+  const GROUPS = [
+    self::ADMIN => "Administrator",
+    self::MODERATOR => "Moderator",
+    self::SUPPORT => "Support",
+  ];
+
   #[MaxLength(32)] public string $name;
   #[MaxLength(10)] public string $color;
 
-  public function __construct(?int $id = null) {
+  public function __construct(?int $id, string $name, string $color) {
     parent::__construct($id);
+    $this->name = $name;
+    $this->color = $color;
   }
 
   public function jsonSerialize(): array {
diff --git a/Core/Objects/DatabaseEntity/Language.class.php b/Core/Objects/DatabaseEntity/Language.class.php
index 23a7a4f..d8cc7e2 100644
--- a/Core/Objects/DatabaseEntity/Language.class.php
+++ b/Core/Objects/DatabaseEntity/Language.class.php
@@ -2,14 +2,17 @@
 
 namespace Core\Objects\DatabaseEntity {
 
-  use Core\Driver\SQL\SQL;
   use Core\Objects\DatabaseEntity\Attribute\MaxLength;
   use Core\Objects\DatabaseEntity\Attribute\Transient;
   use Core\Objects\lang\LanguageModule;
+  use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
   // TODO: language from cookie?
   class Language extends DatabaseEntity {
 
+    const AMERICAN_ENGLISH = 1;
+    const GERMAN_STANDARD = 2;
+
     const LANG_CODE_PATTERN = "/^[a-zA-Z]{2}_[a-zA-Z]{2}$/";
 
     #[MaxLength(5)] private string $code;
diff --git a/Core/Objects/DatabaseEntity/MailQueueItem.class.php b/Core/Objects/DatabaseEntity/MailQueueItem.class.php
new file mode 100644
index 0000000..5abc827
--- /dev/null
+++ b/Core/Objects/DatabaseEntity/MailQueueItem.class.php
@@ -0,0 +1,124 @@
+<?php
+
+namespace Core\Objects\DatabaseEntity;
+
+use Core\API\Mail\Send;
+use Core\Driver\SQL\Expression\CurrentTimeStamp;
+use Core\Objects\Context;
+use Core\Objects\DatabaseEntity\Attribute\DefaultValue;
+use Core\Objects\DatabaseEntity\Attribute\EnumArr;
+use Core\Objects\DatabaseEntity\Attribute\MaxLength;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
+
+class MailQueueItem extends DatabaseEntity {
+
+  protected static array $entityLogConfig = [
+    "update" => true,
+    "delete" => true,
+    "insert" => true,
+    "lifetime" => 30
+  ];
+
+  const STATUS_WAITING = "waiting";
+  const STATUS_SUCCESS = "success";
+  const STATUS_ERROR = "error";
+  const STATUS_ITEMS = [self::STATUS_WAITING, self::STATUS_SUCCESS, self::STATUS_ERROR];
+
+  #[MaxLength(64)]
+  private string $from;
+
+  #[MaxLength(64)]
+  private string $to;
+
+  private string $subject;
+  private string $body;
+
+  #[MaxLength(64)]
+  private ?string $replyTo;
+
+  #[MaxLength(64)]
+  private ?string $replyName;
+
+  #[MaxLength(64)]
+  private ?string $gpgFingerprint;
+
+  #[EnumArr(self::STATUS_ITEMS)]
+  #[DefaultValue(self::STATUS_WAITING)]
+  private string $status;
+
+  #[DefaultValue(5)]
+  private int $retryCount;
+
+  #[DefaultValue(CurrentTimeStamp::class)]
+  private \DateTime $nextTry;
+
+  private ?string $errorMessage;
+
+  public function __construct(string $fromMail, string $toMail, string $subject, string $body,
+                              ?string $replyTo, ?string $replyName, ?string $gpgFingerprint) {
+    parent::__construct();
+    $this->from = $fromMail;
+    $this->to = $toMail;
+    $this->subject = $subject;
+    $this->body = $body;
+    $this->replyTo = $replyTo;
+    $this->replyName = $replyName;
+    $this->gpgFingerprint = $gpgFingerprint;
+    $this->retryCount =  5;
+    $this->nextTry = new \DateTime();
+    $this->errorMessage = null;
+    $this->status = self::STATUS_WAITING;
+  }
+
+  public function jsonSerialize(): array {
+    return [
+      "id" => $this->getId(),
+      "from" => $this->from,
+      "to" => $this->to,
+      "gpgFingerprint" => $this->gpgFingerprint,
+      "subject" => $this->subject,
+      "message" => $this->body,
+      "status" => $this->status,
+      "reply" => [
+        "to" => $this->replyTo,
+        "name" => $this->replyName,
+      ],
+      "retryCount" => $this->retryCount,
+      "nextTry" => $this->nextTry->getTimestamp(),
+      "errorMessage" => $this->errorMessage,
+    ];
+  }
+
+  public function send(Context $context): bool {
+
+    $args = [
+      "to" => $this->to,
+      "subject" => $this->subject,
+      "body" => $this->body,
+      "replyTo" => $this->replyTo,
+      "replyName" => $this->replyName,
+      "gpgFingerprint" => $this->gpgFingerprint,
+      "async" => false
+    ];
+
+    $req = new Send($context);
+    $success = $req->execute($args);
+    $this->errorMessage = $req->getLastError();
+
+    $delay = [0, 720, 360, 60, 30, 1];
+    $minutes = $delay[max(0, min(count($delay) - 1, $this->retryCount))];
+    if ($this->retryCount > 0) {
+      $this->retryCount--;
+      $this->nextTry = (new \DateTime())->modify("+$minutes minute");
+    } else if (!$success) {
+      $this->status = self::STATUS_ERROR;
+    }
+
+    if ($success) {
+      $this->status = self::STATUS_SUCCESS;
+    }
+
+    $this->save($context->getSQL());
+    return $success;
+  }
+}
\ No newline at end of file
diff --git a/Core/Objects/DatabaseEntity/News.class.php b/Core/Objects/DatabaseEntity/News.class.php
index b0c5f07..cf738b7 100644
--- a/Core/Objects/DatabaseEntity/News.class.php
+++ b/Core/Objects/DatabaseEntity/News.class.php
@@ -6,6 +6,7 @@ use Core\API\Parameter\Parameter;
 use Core\Driver\SQL\Expression\CurrentTimeStamp;
 use Core\Objects\DatabaseEntity\Attribute\DefaultValue;
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
 class News extends DatabaseEntity {
 
@@ -16,6 +17,7 @@ class News extends DatabaseEntity {
 
   public function __construct(?int $id = null) {
     parent::__construct($id);
+    $this->publishedAt = new \DateTime();
   }
 
   public function jsonSerialize(): array {
diff --git a/Core/Objects/DatabaseEntity/Notification.class.php b/Core/Objects/DatabaseEntity/Notification.class.php
deleted file mode 100644
index 563226b..0000000
--- a/Core/Objects/DatabaseEntity/Notification.class.php
+++ /dev/null
@@ -1,30 +0,0 @@
-<?php
-
-namespace Core\Objects\DatabaseEntity;
-
-use Core\API\Parameter\Parameter;
-use Core\Driver\SQL\Expression\CurrentTimeStamp;
-use Core\Objects\DatabaseEntity\Attribute\DefaultValue;
-use Core\Objects\DatabaseEntity\Attribute\Enum;
-use Core\Objects\DatabaseEntity\Attribute\MaxLength;
-
-class Notification extends DatabaseEntity {
-
-  #[Enum('default', 'message', 'warning')] private string $type;
-  #[DefaultValue(CurrentTimeStamp::class)] private \DateTime $createdAt;
-  #[MaxLength(32)] public string $title;
-  #[MaxLength(256)] public string $message;
-
-  public function __construct(?int $id = null) {
-    parent::__construct($id);
-  }
-
-  public function jsonSerialize(): array {
-    return [
-      "id" => $this->getId(),
-      "createdAt" => $this->createdAt->format(Parameter::DATE_TIME_FORMAT),
-      "title" => $this->title,
-      "message" => $this->message
-    ];
-  }
-}
\ No newline at end of file
diff --git a/Core/Objects/DatabaseEntity/Session.class.php b/Core/Objects/DatabaseEntity/Session.class.php
index 4fae092..663f8db 100644
--- a/Core/Objects/DatabaseEntity/Session.class.php
+++ b/Core/Objects/DatabaseEntity/Session.class.php
@@ -10,6 +10,7 @@ use Core\Objects\DatabaseEntity\Attribute\DefaultValue;
 use Core\Objects\DatabaseEntity\Attribute\Json;
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
 use Core\Objects\DatabaseEntity\Attribute\Transient;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
 class Session extends DatabaseEntity {
 
diff --git a/Core/Objects/DatabaseEntity/SystemLog.class.php b/Core/Objects/DatabaseEntity/SystemLog.class.php
index 8e14645..fffb258 100644
--- a/Core/Objects/DatabaseEntity/SystemLog.class.php
+++ b/Core/Objects/DatabaseEntity/SystemLog.class.php
@@ -7,6 +7,7 @@ use Core\Driver\SQL\Expression\CurrentTimeStamp;
 use Core\Objects\DatabaseEntity\Attribute\DefaultValue;
 use Core\Objects\DatabaseEntity\Attribute\Enum;
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
 class SystemLog extends DatabaseEntity {
 
diff --git a/Core/Objects/DatabaseEntity/TwoFactorToken.class.php b/Core/Objects/DatabaseEntity/TwoFactorToken.class.php
index 1351d7a..77c7049 100644
--- a/Core/Objects/DatabaseEntity/TwoFactorToken.class.php
+++ b/Core/Objects/DatabaseEntity/TwoFactorToken.class.php
@@ -7,6 +7,7 @@ use Core\Objects\DatabaseEntity\Attribute\Enum;
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
 use Core\Objects\TwoFactor\KeyBasedTwoFactorToken;
 use Core\Objects\TwoFactor\TimeBasedTwoFactorToken;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
 abstract class TwoFactorToken extends DatabaseEntity {
 
diff --git a/Core/Objects/DatabaseEntity/User.class.php b/Core/Objects/DatabaseEntity/User.class.php
index 4b21130..fd7ff0c 100644
--- a/Core/Objects/DatabaseEntity/User.class.php
+++ b/Core/Objects/DatabaseEntity/User.class.php
@@ -2,14 +2,13 @@
 
 namespace Core\Objects\DatabaseEntity;
 
-use Core\Driver\SQL\Condition\Compare;
 use Core\Driver\SQL\Expression\CurrentTimeStamp;
-use Core\Driver\SQL\Join;
 use Core\Driver\SQL\SQL;
 use Core\Objects\DatabaseEntity\Attribute\DefaultValue;
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
-use Core\Objects\DatabaseEntity\Attribute\Transient;
+use Core\Objects\DatabaseEntity\Attribute\Multiple;
 use Core\Objects\DatabaseEntity\Attribute\Unique;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
 class User extends DatabaseEntity {
 
@@ -20,30 +19,16 @@ class User extends DatabaseEntity {
   #[MaxLength(64)] public ?string $profilePicture;
   private ?\DateTime $lastOnline;
   #[DefaultValue(CurrentTimeStamp::class)] public \DateTime $registeredAt;
-  public bool $confirmed;
-  #[DefaultValue(1)] public Language $language;
+  #[DefaultValue(false)] public bool $confirmed;
+  #[DefaultValue(Language::AMERICAN_ENGLISH)] public Language $language;
   public ?GpgKey $gpgKey;
   private ?TwoFactorToken $twoFactorToken;
 
-  #[Transient] private array $groups;
+  #[Multiple(Group::class)]
+  public array $groups;
 
   public function __construct(?int $id = null) {
     parent::__construct($id);
-    $this->groups = [];
-  }
-
-  public function postFetch(SQL $sql, array $row) {
-    parent::postFetch($sql, $row);
-    $this->groups = [];
-
-    $groups = Group::findAllBuilder($sql)
-      ->addJoin(new Join("INNER", "UserGroup", "UserGroup.group_id", "Group.id"))
-      ->where(new Compare("UserGroup.user_id", $this->id))
-      ->execute();
-
-    if ($groups) {
-      $this->groups = $groups;
-    }
   }
 
   public function getUsername(): string {
diff --git a/Core/Objects/DatabaseEntity/UserToken.class.php b/Core/Objects/DatabaseEntity/UserToken.class.php
index 0c41fff..5afbfaa 100644
--- a/Core/Objects/DatabaseEntity/UserToken.class.php
+++ b/Core/Objects/DatabaseEntity/UserToken.class.php
@@ -6,6 +6,7 @@ use Core\Driver\SQL\SQL;
 use Core\Objects\DatabaseEntity\Attribute\DefaultValue;
 use Core\Objects\DatabaseEntity\Attribute\EnumArr;
 use Core\Objects\DatabaseEntity\Attribute\MaxLength;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
 
 class UserToken extends DatabaseEntity {
 
diff --git a/Core/constants.php b/Core/constants.php
index fa49bcf..4de7987 100644
--- a/Core/constants.php
+++ b/Core/constants.php
@@ -1,26 +1,5 @@
 <?php
 
-const USER_GROUP_MODERATOR = 1;
-const USER_GROUP_MODERATOR_NAME = "Moderator";
-const USER_GROUP_SUPPORT = 2;
-const USER_GROUP_SUPPORT_NAME = "Support";
-const USER_GROUP_ADMIN = 3;
-const USER_GROUP_ADMIN_NAME = "Administrator";
-
-const DEFAULT_GROUPS = array(
-  USER_GROUP_MODERATOR, USER_GROUP_SUPPORT, USER_GROUP_ADMIN
-);
-
-function GroupName($index) {
-  $groupNames = array(
-    USER_GROUP_MODERATOR => USER_GROUP_MODERATOR_NAME,
-    USER_GROUP_SUPPORT => USER_GROUP_SUPPORT_NAME,
-    USER_GROUP_ADMIN => USER_GROUP_ADMIN_NAME,
-  );
-
-  return ($groupNames[$index] ?? "Unknown Group");
-}
-
 // adapted from https://www.php.net/manual/en/function.http-response-code.php
 const HTTP_STATUS_DESCRIPTIONS = [
   100 => 'Continue',
diff --git a/Core/core.php b/Core/core.php
index 474881a..92b2377 100644
--- a/Core/core.php
+++ b/Core/core.php
@@ -10,12 +10,12 @@ if (is_file($autoLoad)) {
   require_once $autoLoad;
 }
 
-define("WEBBASE_VERSION", "2.2.0");
+define("WEBBASE_VERSION", "2.3.0");
 
 spl_autoload_extensions(".php");
 spl_autoload_register(function ($class) {
   if (!class_exists($class)) {
-    $suffixes = ["", ".class", ".trait"];
+    $suffixes = ["", ".class", ".trait", ".interface"];
     foreach ($suffixes as $suffix) {
       $full_path = WEBROOT . "/" . getClassPath($class, $suffix);
       if (file_exists($full_path)) {
diff --git a/cli.php b/cli.php
index fb5cc70..6d5f6c0 100644
--- a/cli.php
+++ b/cli.php
@@ -94,13 +94,8 @@ function handleDatabase(array $argv) {
   $action = $argv[2] ?? "";
 
   if ($action === "migrate") {
-    $class = $argv[3] ?? null;
-    if (!$class) {
-      _exit("Usage: cli.php db migrate <class name>");
-    }
-
     $sql = connectSQL() or die();
-    applyPatch($sql, $class);
+
   } else if (in_array($action, ["export", "import", "shell"])) {
 
     // database config
diff --git a/test/DatabaseEntity.test.php b/test/DatabaseEntity.test.php
index 6f68789..5875650 100644
--- a/test/DatabaseEntity.test.php
+++ b/test/DatabaseEntity.test.php
@@ -4,8 +4,8 @@ use Core\API\Parameter\Parameter;
 use Core\Driver\SQL\Query\CreateTable;
 use Core\Driver\SQL\SQL;
 use Core\Objects\Context;
-use Core\Objects\DatabaseEntity\DatabaseEntity;
-use Core\Objects\DatabaseEntity\DatabaseEntityHandler;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntity;
+use Core\Objects\DatabaseEntity\Controller\DatabaseEntityHandler;
 use Core\Objects\DatabaseEntity\User;
 
 class DatabaseEntityTest extends \PHPUnit\Framework\TestCase {
@@ -28,8 +28,9 @@ class DatabaseEntityTest extends \PHPUnit\Framework\TestCase {
   }
 
   public function testCreateTable() {
-    $this->assertInstanceOf(CreateTable::class, self::$HANDLER->getTableQuery());
-    $this->assertTrue(self::$HANDLER->createTable());
+    $query = self::$HANDLER->getTableQuery(self::$CONTEXT->getSQL());
+    $this->assertInstanceOf(CreateTable::class, $query);
+    $this->assertTrue($query->execute());
   }
 
   public function testInsertEntity() {
diff --git a/test/Parameter.test.php b/test/Parameter.test.php
index 269469a..61c165b 100644
--- a/test/Parameter.test.php
+++ b/test/Parameter.test.php
@@ -56,6 +56,7 @@ class ParameterTest extends \PHPUnit\Framework\TestCase {
     // optional single value
     $arrayType = new ArrayType("int_array_single", Parameter::TYPE_INT, true);
     $this->assertTrue($arrayType->parseParam(1));
+    $this->assertEquals([1], $arrayType->value);
 
     // mixed values
     $arrayType = new ArrayType("mixed_array", Parameter::TYPE_MIXED);
diff --git a/test/TimeBasedTwoFactorToken.test.php b/test/TimeBasedTwoFactorToken.test.php
index d81a067..a137530 100644
--- a/test/TimeBasedTwoFactorToken.test.php
+++ b/test/TimeBasedTwoFactorToken.test.php
@@ -27,17 +27,4 @@ class TimeBasedTwoFactorTokenTest extends PHPUnit\Framework\TestCase {
       $this->assertEquals($code, $generated, "$code != $generated, at=$seed");
     }
   }
-
-  public function testURL() {
-    $secret = Base32::encode("12345678901234567890");
-    $context = new Context();
-
-    // $context->
-
-    $token = new TimeBasedTwoFactorToken($secret);
-    $siteName = $context->getSettings()->getSiteName();
-    $username = $context->getUser()->getUsername();
-    $url = $token->getUrl($context);
-    $this->assertEquals("otpauth://totp/$username?secret=$secret&issuer=$siteName", $url);
-  }
 }
\ No newline at end of file