Symfony2 entity form type not saving many to many

Question

Problem: My many-to-many relationship is not being saved

The form is rendered correctly with checkboxes, and if I manually add a relation between a datatypegroup and a datatype the checkbox is also checked.

But if I check another datatype and save the form, the new relation is not established. I have tried with and without by_reference in the form, but no luck. The addDatatype method on the datatypegroup entity is not called either. Do I really have to loop the form data and find the entities myself?

FormBuilder:

$builder
        ->add('name', 'text')
        ->add('datatypes', 'entity', array(
            'by_reference' => true,
            'multiple' => true,   
            'expanded' => true,   
            'class'    => 'T\DBundle\Entity\Datatype',
            'query_builder' => function(\Doctrine\ORM\EntityRepository $er) {
                    $qb = $er->createQueryBuilder('d');
                    return $qb->orderBy('d.name', 'ASC');
            }
        ));

DoctrineMapping:

T\DBundle\Entity\DatatypeGroup:
    type: entity
    table: DatatypeGroup
    manyToMany:
        datatypes:
            targetEntity: T\DBundle\Entity\Datatype
            mappedBy: datatypeGroups

T\DBundle\Entity\Datatype:
    type: entity
    table: Datatype

    manyToMany:
        datatypeGroups:
            targetEntity: T\DBundle\Entity\DatatypeGroup
            inversedBy: datatypes
            joinTable:
                name: Datatype_DatatypeGroup
                joinColumns:
                    datatype_id:
                        referencedColumnName: id
                inverseJoinColumns:
                    datatypeGroup_id:
                        referencedColumnName: id

Controller:

/**
 * @Route("/edit/{id}", name="edit")
 * @Secure("ROLE_USER")
 * @Template("TDBundle:DatatypeGroup:form.html.twig")
 */
public function editAction(Request $request, DatatypeGroup $datatypegroup) {
    $editForm = $this->createForm(new DatatypeGroupType(), $datatypegroup);
    $editForm->handleRequest($request);

    if ($editForm->isValid()) {
        /** @var EntityManager $em */
        $em = $this->getDoctrine()->getManager();
        $em->flush();

        return $this->redirect($this->generateUrl("edit",array("id" => $datatypegroup->getId())));
    }

    return array(
        'datatypegroup' => $datatypegroup,
        'form'          => $editForm->createView(),
    );
}

Answer 1

I do not know how to mark this as a duplicate, but I finally found a question answering my question: Symfony2 Doctrine2 Many To Many Form not Saving Entities

The doctrine mapping had to be reversed so that the datatypegroup was the owning type of the relationship.