Symfony2-Doctrine: ManyToMany bi-directionnal relation - orm

Question

I've already search a lot befors asking, even the related topic Symfony2-Doctrine: ManyToMany relation is not saved to database
but still no answer.
I've got this two classes:
class Intervenant extends User
{
/**
* #ManyToMany(targetEntity="iMDEO\DISAASBundle\Entity\Domaine", inversedBy="intervenants", cascade={"persist","merge"})
*/
private $domaines;
/**
* Add domaines
*
* #param Domaine $domaines
*/
public function addDomaine(Domaine $domaines)
{
$this->domaines[] = $domaines;
}
/**
* Get domaines
*
* #return Doctrine\Common\Collections\Collection
*/
public function getDomaines()
{
return $this->domaines;
}
}
class Domaine
{
// ...
/**
* #ORM\ManyToMany(targetEntity="Intervenant", mappedBy="domaines", cascade={"persist","merge"})
*
*/
private $intervenants;
/**
* Add intervenants
*
* #param Intervenant $intervenants
*/
public function addIntervenant(Intervenant $intervenants)
{
$intervenants->addDomaine($this);
$this->intervenants[] = $intervenants;
}
/**
* Get intervenants
*
* #return Doctrine\Common\Collections\Collection
*/
public function getIntervenants()
{
return $this->intervenants;
}
}
When I save an Intervenant, everthing is OK.
But when i save the inverse side Domaine, the changes are not persisted.
Reading Symfony's doc and topics everywhere, I can't find any solution to get a bi-directionnal relation between my two entities.
Here's part of my DomaineController:
$em = $this->getDoctrine()->getEntityManager();
$entity = $em->getRepository('myBundle:Domaine')->find($id);
if (!$entity) {
throw $this->createNotFoundException('Unable to find Domaine entity.');
}
$editForm = $this->createForm(new DomaineType(), $entity);
$deleteForm = $this->createDeleteForm($id);
$request = $this->getRequest();
$editForm->bindRequest($request);
if ($editForm->isValid()) {
$em->persist($entity);
$em->flush();
return $this->indexAction();
}
// ...
My purpose is that when I create/edit an Intervenant, I can choose related Domaine.
And when I create/edit a Domaine, I link every Intervenants in it.
Could you please help me?