Zend Framework:如何在使用DbTable Auth Adapter时检查其他列？

class My_Auth_Adapter_DbTable extends Zend_Auth_Adapter_DbTable
{
    protected $_otherColumns = array();

    public function __construct($db, $tableName, $identityColumn, $credentialColumn, $otherColumns = array())
    {
        parent::__construct($db, $tableName, $identityColumn, $credentialColumn);
        $this->_otherColumns = $otherColumns;
    }

    protected function _authenticateQuerySelect()
    {
        $select = parent::_authenticateQuerySelect();
        foreach ($this->_otherColumns as $column) {
            $select->columns[] = $column;
        }
        return $select;
    }
}

$authAdapter = new My_Auth_Adapter_DbTable($db, 'users', 'username', 'password', array('email', 'last_login'));
$authAdapter->setIdentity('my_username')
             ->setCredential('my_password');
$auth = Zend_Auth::getInstance();
$result = $auth->authenticate($authAdapter);

if ($result->isValid()) {
    $identity = $authAdapter->getResultRowObject(null, array('password'));
    // $identity 现在包含了用户名、密码、电子邮件和最后登录时间的列
}