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