Using a custom UserProviderInterface for Symfony Security - http://www.symfonycentral.com/securing-your-web-application-with-symfony.html

UserRepository.php

<?php

namespace MyCompany\BlogBundle\Entity;

use Doctrine\ORM\EntityRepository;

// Make sure we include UserProviderInterface
use Symfony\Component\Security\Core\User\UserProviderInterface;
use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
use Symfony\Component\Security\Core\Exception\UnsupportedUserException;

class UserRepository implements UserProviderInterface // Add the 'implements' clause
{
    function loadUserByUsername($username)
    {
        // Find and load the user from the database
        $q = $this
            ->createQueryBuilder('u')
            ->where('u.username = :username')
            ->setParameter('username', $username)
            ->getQuery()
        ;
        
        // If we don't find a record, throw an exception, authentication fails
        try {
            $user = $q->getSingleResult();
        } catch (NoResultException $e) {
            throw new UsernameNotFoundException(
                'Unable to find an active admin MyCompanyBlogBundle:User object identified'
            );
        }

        return $user;
    }

    function refreshUser(UserInterface $user)
    {
        $class = get_class($user);
        if (!$this->supportsClass($class)) {
            throw new UnsupportedUserException('User type not supported');
        }

        return $this->loadUserByUsername($user->getUsername());
    }

    function supportsClass($class)
    {
        return $this->getEntityName() === $class || 
            is_subclass_of($class, $this->getEntityName());
    }
}