private function fetchAllUserNames()
 {
     if (empty($this->user_names)) {
         $this->user_names[100] = '';
         foreach ($this->dao->getAllUsers() as $user_info) {
             $this->user_names[$user_info['user_id']] = $user_info['user_name'];
         }
     }
     return $this->user_names;
 }