public function addUsersAction($id, $class, Request $request)
    {
        $object = $this->find($class, $id);
        $users = $this->decodeIdsString($request, 'Claroline\CoreBundle\Entity\User');
        $this->crud->patch($object, 'user', Crud::COLLECTION_ADD, $users);

        return new JsonResponse(
            $this->serializer->serialize($object)
        );
    }