Implement UserProvider in Symfony2

config.yml

services:
  user.provider:
    class: MyApp\MyBundle\DependencyInjection\UserProvider
    arguments: [@doctrine.orm.entity_manager]

security.yml

security:
    providers:
        main:
            id: user.provider

UserProvider.php

<?php
namespace MyApp\MyBundle\DependencyInjection;

use Symfony\Component\Security\Core\User\User;
use Symfony\Component\Security\Core\User\UserInterface;
use Symfony\Component\Security\Core\User\UserProviderInterface;
use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;

class UserProvider implements UserProviderInterface {

	private $db = null;

	public function __construct($db_link) {
		$this->db = $db_link;
	}

	function loadUserByUsername($username) {
		$optionRep = $this->db->getRepository('MyApp\MyBundle\Entity\Option');
/*
There is only a options-database with two users...
*/
		$admin = $optionRep->find('adminName');
		if(!is_null($admin) && $admin->getValue() == $username) {
			$password = $optionRep->find('adminPassword')->getValue();
			return new User($username, $password, array('ROLE_ADMIN'));
		}
		$user = $optionRep->find('partnerName');
		if(!is_null($user) && $user->getValue() == $username) {
			$password = $optionRep->find('partnerPassword')->getValue();
			return new User($username, $password, array('ROLE_USER'));
		}

		throw new UsernameNotFoundException('Benutzername oder Passwort falsch.');
	}

	function loadUser(UserInterface $user) {
		return $this->loadUserByUsername($user->getUsername());
	}

	function supportsClass($class) {

	}

}