public function callback_before_update($callback = null, $override_all = 0)
 {
     if (!$override_all) {
         $this->callback_before_update_ext[] = $callback;
         if ($this->callback_before_update == null) {
             $this->callback_before_update = array($this, 'extended_callback_before_update');
         }
     } else {
         parent::callback_before_update($callback);
     }
     return $this;
 }