diff --git a/Models/Event.php b/Models/Event.php
index eac70f3..afbc923 100644
--- a/Models/Event.php
+++ b/Models/Event.php
@@ -14,8 +14,8 @@ declare(strict_types=1);
 
 namespace Modules\Calendar\Models;
 
+use Modules\Admin\Models\NullAccount;
 use phpOMS\Account\Account;
-use phpOMS\Account\NullAccount;
 use phpOMS\Stdlib\Base\Location;
 
 /**
@@ -63,10 +63,10 @@ class Event
     /**
      * Creator.
      *
-     * @var int|\Modules\Admin\Models\Account
+     * @var Account
      * @since 1.0.0
      */
-    private $createdBy = 0;
+    private Account $createdBy;
 
     /**
      * Event type.
@@ -91,10 +91,10 @@ class Event
     /**
      * Schedule
      *
-     * @var int|Schedule
+     * @var Schedule
      * @since 1.0.0
      */
-    private $schedule;
+    private Schedule $schedule;
 
     /**
      * Location of the event.
@@ -127,6 +127,7 @@ class Event
      */
     public function __construct()
     {
+        $this->createdBy = new NullAccount();
         $this->createdAt = new \DateTime('now');
         $this->location  = new Location();
         $this->schedule  = new Schedule();
@@ -247,11 +248,11 @@ class Event
     }
 
     /**
-     * @return int|\Modules\Admin\Models\Account
+     * @return Account
      *
      * @since 1.0.0
      */
-    public function getCreatedBy()
+    public function getCreatedBy() : Account
     {
         return $this->createdBy;
     }
@@ -259,13 +260,13 @@ class Event
     /**
      * Set creator
      *
-     * @param int $createdBy Creator
+     * @param Account $createdBy Creator
      *
      * @return void
      *
      * @since 1.0.0
      */
-    public function setCreatedBy(int $createdBy) : void
+    public function setCreatedBy(Account $createdBy) : void
     {
         $this->createdBy = $createdBy;
 
@@ -339,11 +340,11 @@ class Event
     }
 
     /**
-     * @return int|Schedule
+     * @return Schedule
      *
      * @since 1.0.0
      */
-    public function getSchedule()
+    public function getSchedule() : Schedule
     {
         return $this->schedule;
     }
diff --git a/Models/EventMapper.php b/Models/EventMapper.php
index a5f067a..b344dd8 100644
--- a/Models/EventMapper.php
+++ b/Models/EventMapper.php
@@ -14,6 +14,7 @@ declare(strict_types=1);
 
 namespace Modules\Calendar\Models;
 
+use Modules\Admin\Models\AccountMapper;
 use phpOMS\DataStorage\Database\DataMapperAbstract;
 
 /**
@@ -41,7 +42,7 @@ final class EventMapper extends DataMapperAbstract
         'calendar_event_status'      => ['name' => 'calendar_event_status',      'type' => 'int',          'internal' => 'status'],
         'calendar_event_schedule'    => ['name' => 'calendar_event_schedule',    'type' => 'int',          'internal' => 'schedule'],
         'calendar_event_calendar'    => ['name' => 'calendar_event_calendar',    'type' => 'int',          'internal' => 'calendar'],
-        'calendar_event_created_by'  => ['name' => 'calendar_event_created_by',  'type' => 'int',          'internal' => 'createdBy'],
+        'calendar_event_created_by'  => ['name' => 'calendar_event_created_by',  'type' => 'int',          'internal' => 'createdBy', 'readonly' => true],
         'calendar_event_created_at'  => ['name' => 'calendar_event_created_at',  'type' => 'DateTime',     'internal' => 'createdAt', 'readonly' => true],
     ];
 
@@ -58,6 +59,19 @@ final class EventMapper extends DataMapperAbstract
         ],
     ];
 
+    /**
+     * Belongs to.
+     *
+     * @var array<string, array{mapper:string, self:string}>
+     * @since 1.0.0
+     */
+    protected static array $belongsTo = [
+        'createdBy' => [
+            'mapper' => AccountMapper::class,
+            'self'   => 'calendar_event_created_by',
+        ],
+    ];
+
     /**
      * Primary table.
      *
diff --git a/Models/Schedule.php b/Models/Schedule.php
index 3ac591b..ab1f930 100644
--- a/Models/Schedule.php
+++ b/Models/Schedule.php
@@ -14,6 +14,8 @@ declare(strict_types=1);
 
 namespace Modules\Calendar\Models;
 
+use Modules\Admin\Models\Account;
+use Modules\Admin\Models\NullAccount;
 use phpOMS\Stdlib\Base\Exception\InvalidEnumValue;
 
 /**
@@ -125,10 +127,10 @@ class Schedule
     /**
      * Created by.
      *
-     * @var int|\Modules\Admin\Models\Account
+     * @var Account
      * @since 1.0.0
      */
-    private $createdBy = 0;
+    private Account $createdBy;
 
     /**
      * Constructor.
@@ -137,6 +139,7 @@ class Schedule
      */
     public function __construct()
     {
+        $this->createdBy = new NullAccount();
         $this->createdAt = new \DateTime('now');
         $this->start     = new \DateTime('now');
         $this->end       = new \DateTime('now');
@@ -170,7 +173,7 @@ class Schedule
      *
      * @since 1.0.0
      */
-    public function setStatus(int $status)
+    public function setStatus(int $status) : self
     {
         if (!ScheduleStatus::isValidValue($status)) {
             throw new InvalidEnumValue($status);
@@ -198,7 +201,7 @@ class Schedule
      *
      * @since 1.0.0
      */
-    public function setFreqType(int $freqType)
+    public function setFreqType(int $freqType) : self
     {
         if (!FrequencyType::isValidValue($freqType)) {
             throw new InvalidEnumValue($freqType);
@@ -226,7 +229,7 @@ class Schedule
      *
      * @since 1.0.0
      */
-    public function setIntervalType(int $intervalType)
+    public function setIntervalType(int $intervalType) : self
     {
         if (!IntervalType::isValidValue($intervalType)) {
             throw new InvalidEnumValue($intervalType);
@@ -254,7 +257,7 @@ class Schedule
      *
      * @since 1.0.0
      */
-    public function setFrequencyRelative(int $relativeInterval)
+    public function setFrequencyRelative(int $relativeInterval) : self
     {
         if (!FrequencyRelative::isValidValue($relativeInterval)) {
             throw new InvalidEnumValue($relativeInterval);
@@ -272,7 +275,7 @@ class Schedule
      *
      * @since 1.0.0
      */
-    public function setFreqInterval(int $freqInterval)
+    public function setFreqInterval(int $freqInterval) : self
     {
         if (!FrequencyInterval::isValidValue($freqInterval)) {
             throw new InvalidEnumValue($freqInterval);
@@ -310,7 +313,7 @@ class Schedule
      *
      * @since 1.0.0
      */
-    public function setRecurrenceFactor(int $recurrence)
+    public function setRecurrenceFactor(int $recurrence) : self
     {
         $this->recurrenceFactor = $recurrence;
 
@@ -334,7 +337,7 @@ class Schedule
      *
      * @since 1.0.0
      */
-    public function setStart(\DateTime $start)
+    public function setStart(\DateTime $start) : self
     {
         $this->start = $start;
 
@@ -394,13 +397,13 @@ class Schedule
     }
 
     /**
-     * @param int $creator Creator
+     * @param Account $creator Creator
      *
      * @return $this
      *
      * @since 1.0.0
      */
-    public function setCreatedBy(int $creator)
+    public function setCreatedBy(Account $creator) : self
     {
         $this->createdBy = $creator;
 
@@ -408,11 +411,11 @@ class Schedule
     }
 
     /**
-     * @return int|\Modules\Admin\Models\Account
+     * @return Account
      *
      * @since 1.0.0
      */
-    public function getCreatedBy()
+    public function getCreatedBy() : Account
     {
         return $this->createdBy;
     }
diff --git a/Models/ScheduleMapper.php b/Models/ScheduleMapper.php
index 3649628..5adc4ba 100644
--- a/Models/ScheduleMapper.php
+++ b/Models/ScheduleMapper.php
@@ -14,6 +14,7 @@ declare(strict_types=1);
 
 namespace Modules\Calendar\Models;
 
+use Modules\Admin\Models\AccountMapper;
 use phpOMS\DataStorage\Database\DataMapperAbstract;
 
 /**
@@ -45,7 +46,20 @@ final class ScheduleMapper extends DataMapperAbstract
         'schedule_duration'               => ['name' => 'schedule_duration',               'type' => 'int',      'internal' => 'duration'],
         'schedule_end'                    => ['name' => 'schedule_end',                    'type' => 'DateTime', 'internal' => 'end'],
         'schedule_created_at'             => ['name' => 'schedule_created_at',             'type' => 'DateTime', 'internal' => 'createdAt', 'readonly' => true],
-        'schedule_created_by'             => ['name' => 'schedule_created_by',             'type' => 'int',      'internal' => 'createdBy'],
+        'schedule_created_by'             => ['name' => 'schedule_created_by',             'type' => 'int',      'internal' => 'createdBy', 'readonly' => true],
+    ];
+
+    /**
+     * Belongs to.
+     *
+     * @var array<string, array{mapper:string, self:string}>
+     * @since 1.0.0
+     */
+    protected static array $belongsTo = [
+        'createdBy' => [
+            'mapper' => AccountMapper::class,
+            'self'   => 'schedule_created_by',
+        ],
     ];
 
     /**