private function findPreviousUpdate($assignmentId)
 {
     $previousAssignmentIdCached = Cache::get('previous_assignment_id_for_' . $assignmentId);
     if ($previousAssignmentIdCached !== null) {
         return $previousAssignmentIdCached;
     } else {
         $ascUpdates = AssignmentUpdate::where('assignment_id', $assignmentId)->orderBy('created_at', 'DESC')->get()->last();
         return $ascUpdates;
     }
 }