ORM\Id\AssignedGenerator doesn't work with cascade persist and auto incremented id

                        if (!$em->getUnitOfWork()->isInIdentityMap($value)) {
                            throw ORMException::entityMissingForeignAssignedId($entity, $value);
                        }

class User {

    /**
     * @var int
     * @ORM\Id
     * @ORM\Column(type="integer", unique=true, nullable=false)
     * @ORM\GeneratedValue
     */
    protected $user_id;


    /**
     * @var \Doctrine\ORM\PersistentCollection
     * @ORM\OneToMany(targetEntity="Role", mappedBy="user", cascade={"persist", "remove", "merge"}, orphanRemoval=true)
     */
    protected $roles;
}

class Role {
    /**
     * @var User
     * @ORM\Id
     * @ORM\ManyToOne(targetEntity="User", inversedBy="roles")
     * @ORM\JoinColumn(name="user_id", referencedColumnName="user_id")
     */
    protected $user;

    ...
}

$Role = new Role();
$Role->setName("ROLE_SOME");
$User = new User();
$User->getRoles()->add($Role);
$em->persist($User);

Doctrine\ORM\ORMException: Entity of type Role has identity through a foreign entity User, however this entity has no ientity itself. You have to call EntityManager#persist() on the related entity and make sure it an identifier was generated before trying to persist 'Role'. In case of Post Insert ID Generation (such as MySQL Auto-Increment or PostgreSQL SERIAL) this means you have to call EntityManager#flush() between both persist operations.