protected function _getIdentifier(\Zend_EventManager_EventDescription $event)
    {
        $params = $event->getParams();
        if(!isset($params['id'])) {
            throw new \Zend_EventManager_Exception_InvalidArgumentException('Missing param id.');
        }

        return md5($this->_getClassName($event->getTarget()) . '-' . $params['id']);
    }