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be/app/Observers/SubTaskObserver.php

@@ -19,6 +19,9 @@ class SubTaskObserver
 
     public function updating(SubTask $subTask)
     {
+        // to prevent updating created_by field. this field cannot be updated once created.
+        $subTask->created_by = $subTask->getOriginal('created_by');
+
         $subTask->tracked_time = $subTask->timesheets->sum('time');
     }