Symfony 2.7, Doctrine cascade remove - symfony

Question

I try to use the doctrine cascade={"remove"} but it doesn't work.
The situation :
I have announces that contains many pictures (one picture can refer to only one announce). I want that when i delete the announce from the DB it's remove the pictures too.
Here is my code :
Entity announce :
/**
* Announce
*
* #ORM\Table()
* #ORM\Entity(repositoryClass="AppBundle\Entity\Announce\AnnounceRepository")
*/
class Announce
{
/**
* #ORM\OneToOne(targetEntity="AppBundle\Entity\Announce\PictureAnnounce", inversedBy="coverAnnounce")
* #ORM\JoinColumn(name="cover_picture_id", referencedColumnName="id", nullable=true)
*/
private $coverPicture;
/**
* #ORM\OneToMany(targetEntity="AppBundle\Entity\Announce\PictureAnnounce", mappedBy="announce", cascade={"remove"})
*/
private $pictureAnnounces;
}
Entity pictureAnnounce :
/**
* AppBundle\Entity\Announce\PictureAnnounce
*
* #ORM\Table(name="picture_announce")
* #ORM\Entity(repositoryClass="AppBundle\Entity\Announce\PictureAnnounceRepository")
* #ORM\HasLifecycleCallbacks
*/
class PictureAnnounce
{
/**
* #ORM\OneToOne(targetEntity="AppBundle\Entity\Announce\Announce", mappedBy="coverPicture", cascade={"persist"})
*/
private $coverAnnounce;
/**
* #var Announce $announce
*
* #ORM\ManyToOne(targetEntity="AppBundle\Entity\Announce\Announce", inversedBy="pictureAnnounces")
* #ORM\JoinColumn(name="announce_id", referencedColumnName="id", nullable=false)
*/
private $announce;
The code for the delete (i set coverPicture to null so i don't have a foreign key error when removing)
$announce->setCoverPicture(null);
$em->flush();
$em->remove($announce);
$em->flush();
But i have an error on foreign key
SQLSTATE[23000]: Integrity constraint violation: 1451 Cannot delete or
update a parent row: a foreign key constraint fails
PS : I can't found how to render correctly my code :s
PS : This workaround works, but isn't it the way the cascade work?
if(count($announce->getPictureAnnounces()) > 0) {
$pictureAnnounces = $announce->getPictureAnnounces();
foreach ($pictureAnnounces as $pictureAnnounce) {
$em->remove($pictureAnnounce);
}
$em->flush();
} // Then remove the announce