Account.php

<?php
/**
 * @UniqueEntity(fields={"login"}, groups={"registration"}, message="login already taken")
 */
class Account {
    /**
     * @var string
     *
     * @ORM\Column(name="login", type="string", length=255, nullable=false, unique=true)
     * @Assert\NotBlank()
     */
    private $login;
}

AccountType.php

<?php
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Entity\Account',
            'validation_groups' => array('registration'),
        ));