Как сделать валидацию require с ИЛИ на yii?

public function rules() {
    return array(
        array('username','either','other'=>'email'),
    );
}
public function either($attribute_name, $params)
{
    $field1 = $this->getAttributeLabel($attribute_name);
    $field2 = $this->getAttributeLabel($params['other']);
    if (empty($this->$attribute_name) && empty($this->$params['other'])) {
        $this->addError($attribute_name, Yii::t('user', "either {$field1} or {$field2} is required."));
        return false;
    }
    return true;
}