Symfony sfDoctrineGuardPlugin custom login query

I use symfony sfDoctrineGuardPlugin to manage authentication for both frontend users and backend users. It's fine, except that I don't want frontend users to be able to login to the backend app. I can setup credentials, but credentials are checked after a user gets authenticated. What I want is to have sigin in form to never validate for a user, that is not in a backend group. How can I do this?

I think I found a better solution. sfDoctrineGuard plugin has its own post validator that checks for an optional callable for user retrival.

//app.yml
all:
  sf_guard_plugin:
    retrieve_by_username_callable: sfGuardUser::getForBackend

//sfGuardUser.class.php

  public static function getForBackend($username)
  {
    $query = Doctrine::getTable('sfGuardUser')->createQuery('u')
      ->leftJoin('u.Groups g')
      ->leftJoin('g.Permissions p')
      ->where('u.username = ? OR u.email_address = ?', array($username, $username))
      ->addWhere('u.is_active = ?', true)
      ->addWhere('p.name = ?', 'backend');

    return $query->fetchOne();
  }