Insert foreign key in Symfony2 in OneToMany relation - symfony

Question

I have a problem with adding data to the db. I have two entities InternalDocument and InternalDocumentProduct in OneToMany relation.
In InternalDocument:
/**
* #ORM\OneToMany(targetEntity="InternalDocumentProduct", mappedBy="document", cascade={"all"})
**/
protected $products;
In InternalDocumentProduct
/**
* #ORM\ManyToOne(targetEntity="InternalDocument", inversedBy="products")
* #ORM\JoinColumn(name="document_id", referencedColumnName="id")
* */
protected $document;
When I create new InternalDocument I need to insert InternalDocumentProduct too. But When i call persist() method, InternalDocumentProduct is saved without document_id field. It's null. This is my createForm() method in InternalDocumentController:
/**
* Creates a new InternalDocument entity.
*
*/
public function createAction(Request $request)
{
$entity = new InternalDocument();
$form = $this->createCreateForm($entity);
$form->handleRequest($request);
$em = $this->getDoctrine()->getManager();
if ($form->isValid()) {
$em->persist($entity);
$em->flush();
$em = $this->getDoctrine()->getManager();
foreach($entity->getProducts() as $p) {
$em->persist($p);
}
$em->flush();
return $this->redirect($this->generateUrl('acme_warehouse_documents'));
}
return $this->render('AcmeWarehouseBundle:InternalDocument:new.html.twig', array(
'entity' => $entity,
'form' => $form->createView(),
));
}
Can anyone help me?
EDIT:
I resolve this problem. I modified createAction method:
/**
* Creates a new InternalDocument entity.
*
*/
public function createAction(Request $request)
{
$entity = new InternalDocument();
$form = $this->createCreateForm($entity);
$form->handleRequest($request);
$em = $this->getDoctrine()->getManager();
if ($form->isValid()) {
foreach($entity->getProducts() as $p) {
$p->setDocument($entity);
}
$em->persist($entity);
$em->flush();
return $this->redirect($this->generateUrl('acme_warehouse_documents'));
}
return $this->render('AcmeWarehouseBundle:InternalDocument:new.html.twig', array(
'entity' => $entity,
'form' => $form->createView(),
));
}