Capture Entity serialized in JSON with symfony form

I am using fos_rest in symfony 2.8. I have an Angular client that sends a serialized entity in JSON in order to persist it in the database. It has three related entities.

Ex:

{"auto":
        {"usuario":{"id":1},
         "modelo":{"id":1},
         "color":{"id":1},
         "nombre":"Mi Auto",
         "kmActual":5300,
         "anio":2016}
}

To capture it I have a formType like the following:

class AutoType extends AbstractType
{

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder->add('nombre', TextType::class)
            ->add('kmActual', IntegerType::class)
            ->add('anio', IntegerType::class)
            ->add('usuario', UsuarioType::class)
            ->add('modelo', ModeloType::class)
            ->add('color', ColorType::class)
            ->add('id');
}
public function configureOptions(OptionsResolver $resolver)
{
    $resolver->setDefaults(array(
        'data_class' => 'AppBundle\Entity\Auto',
        'csrf_protection' => false,
        'allow_extra_fields' => true,
    ));
}

I also have three forms: UserType, ModelType and ColorType with structures very similar to AutoType.

In the controller I have the following:

/**
* Collection post action
* @var Request $request
* @return View|array
*/
public function cpostAction(Request $request)
{
   $entity = new Auto();
   $form = $this->createForm(AutoType::class, $entity);
    $form->handleRequest($request);  
   if ($form->isValid()) {
       $entity = $form->getData();
       $em = $this->getDoctrine()->getManager();
   );
       $em->merge($entity);
       $em->flush();           
       return $this->redirectView(
               $this->generateUrl(
                   'get_auto',
                   array('idAuto' => $entity->getId())
                   ),
                   Response::HTTP_CREATED
               );
   }
   return $this->view($form->getErrors(), Response::HTTP_BAD_REQUEST);
   }

I used merge because when trying to do persist I tried to insert user, model and color as new records. Doing etso inserts in the database but does not update the entity with the id inserted.

What am I doing wrong?