如何在symfony2中验证实体集合中的唯一实体

use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;

/**
 * @ORM\Entity
 * @ORM\Table(name="users")
 * @UniqueEntity("username")
 */
class User
{
    // ...

    /**
     * @ORM\Column(type="string", unique=true)
     */
    private $username;

    // ...
}

use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;

/**
 * @UniqueEntity(fields="username", message="用户名已存在")
 */
class UserValidator
{
    // ...
}

use Symfony\Component\Validator\Validator\ValidatorInterface;

class UserController
{
    // ...

    public function createAction(Request $request, ValidatorInterface $validator)
    {
        // ...

        $errors = $validator->validate($user);

        if (count($errors) > 0) {
            // 处理验证错误
        }

        // ...
    }

    // ...
}