Symfony2 Forms: create new or select existing

I have and entity A wich relates oneToMany with entity B. I want the user to have the option of selecting from existing B entities or create a new one on the form of type A. So far I have this on my form:

->add('ExistingB', 'entity', array(
            'class' => 'AppBundle\Entity\B',
            'required'    => false,
            'property_path' => 'B',
            'mapped' => false,
        ))

 ->add('newB',  new BType(), array(
            'required'    => false,
            'property_path' => 'B',
            'mapped' => false,
        ));

 $builder->addEventListener(
     FormEvents::POST_SUBMIT , function (FormEvent $event) {
          $formB= $event->getForm()->get('newB');

         if ($formB->get('name')->getData() != null){
                 //here i need to somehow say to the form that it needs to set mapped true 
                //to the formB field so it can create a new entity and update the relationship
         }else{
                  //here I need to do the same but with the ExistingB field
                }
         }
    );`

I cant find how to change the mapped atribute, and the times I got it, it doenst create the entity. I suppose thats becasuse in the post_submit event its too late for changes on the fields since the data is already downloaded to the A entity. Buuut.. if I use the pre_submit event, then I cant get the data of the child formB, since it always gives me null when I ask for it.

So... where is my big mistake? Somebody can show me another way to deal with new or existing feature in synfony2 forms. I really cant believe that it could be that hard to implement so common behavior.

Thanks so much in advance

you can use pre submit event, you just have to do this like this: https://github.com/LPodolski/selectOrCreateOptionForm/blob/master/src/AppBundle/Form/ItemType.php#L36

full project that demonstrates how this should work: https://github.com/LPodolski/selectOrCreateOptionForm