Using custom constraints

User.php

<?php

namespace Acme\DemoBundle\Entity;

use Symfony\Component\Security\Core\User\UserInterface;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;

/**
 * @ORM\Entity
 * @UniqueEntity(fields="username", message="The username is already used")
 */
abstract class User implements UserInterface
{
    /**
     * @var string
     * @ORM\Column(unique=true)
     */
    protected $username;

validation.xml

<!-- First solution: FQCN -->
<constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping
        http://symfony.com/schema/dic/services/constraint-mapping-1.0.xsd">

    <class name="Acme\DemoBundle\Entity\User">

        <constraint name="Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity">
            <option name="fields">username</option>
            <option name="message">The username is already used.</option>
        </constraint>
    </class>

</constraint-mapping>

<!-- Second solution: using a prefix -->
<constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping
        http://symfony.com/schema/dic/services/constraint-mapping-1.0.xsd">

    <namespace prefix="orm">Symfony\Bridge\Doctrine\Validator\Constraints\</namespace>

    <class name="Acme\DemoBundle\Entity\User">

        <constraint name="orm:UniqueEntity">
            <option name="fields">username</option>
            <option name="message">The username is already used.</option>
        </constraint>
    </class>

</constraint-mapping>

validation.yaml

# First solution: FQCN
Acme\DemoBundle\Entity\User:
    constraints:
        - Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity: { fields: username, message: The username is already used. }

# Second solution: using a prefix
namespaces:
    orm: Symfony\Bridge\Doctrine\Validator\Constraints\

Acme\DemoBundle\Entity\User:
    constraints:
        - orm:UniqueEntity: { fields: username, message: The username is already used. }