Symfony forms -- removing an optional one-to-one relationship in an embedded form - forms

Question

I'm trying to use Symfony Forms to edit an existing entity that has a one-to-one relationship with another entity that is optional. I want to make it so that if it receives nothing for the associated entity, it will delete the entity.
Here's the gist of the code I've written:
class AType extends AbstractType
{
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder->add('BType', new BType(), ['required' => false]);
}
}
The A entity's association with B:
class A
{
/**
* #ORM\OneToOne(
* targetEntity="B",
* mappedBy="a",
* cascade={"persist", "remove"},
* orphanRemoval=true
* )
* #Assert\Valid()
*/
private $b;
....
When I try binding the form with the data, even though nothing was submitted for B, it'll make validation errors that B's fields are blank, because there is an existing B entity associated with A.
I've also looked at FormEvents, but I have not been able to find a way to fix the problem that way either -- example of what I've tried adding to AType's buildForm:
$builder->addEventListener(FormEvents::POST_SUBMIT, function(FormEvent $event) {
$a = $event->getData();
if($a->getType() != 'Package') {
$a->setB(null);
}
});
Even with this additional code, the errors regarding B are still appearing.
Update:
I think I've gotten closer, but though I can tell it's getting into the new code correctly, the B entity still doesn't ultimately get deleted, I think because despite the $a->setB(null), for $a = $form->getData(); in the controller, $a->getB(); still returns data (i.e. editing the entity in AType's POST_SUBMIT seems to only change that data locally?)
New Code:
$builder->addEventListener(FormEvents::PRE_SUBMIT, function(FormEvent $event) {
$data = $event->getData();
if(!isset($data['B'])) {
$event->getForm()->remove('B');
}
});
$builder->addEventListener(FormEvents::POST_SUBMIT, function(FormEvent $event) {
$a = $event->getData();
if(!$event->getForm()->has('B') && $a->getB()) {
$a->setB(null);
}
});
I'm really at a loss because while $a->setB(null) does not perpetuate to the controller, if I create a property $a->test and set that in the same location, that does perpetuate to the controller.
Update 2
As per comments, I'm adding some code from Entity B and the controller:
class B
{
/**
* #var \A
*
* #ORM\GeneratedValue(strategy="NONE")
* #ORM\OneToOne(targetEntity="A", inversedBy="b", cascade={"persist"})
* #ORM\JoinColumns({
* #ORM\JoinColumn(name="a_id", referencedColumnName="id")
* })
*/
private $a;
As for the controller, it gets more complicated as I'm actually dealing with a 2 deep relation: C hasMany A hasOne B. It's also not using form inputs, but rather formatted json. I'm using http://jmsyst.com/bundles/JMSSerializerBundle/master/installation to serialize the output back into json, as well as a helper function to serialize any error messages.
//Where $c is either a new C entity or an existing C entity to be edited
private function handleForm($c)
{
$json = $this->getJsonFromRequest();
if (false === $json) {
throw new \Exception('Invalid JSON');
}
$form = $this->createForm(new CType(), $c);
$em = $this->getDoctrine()->getEntityManager();
if($form->bind(json_decode($json, true)) && $form->isValid()) {
$c = $form->getData();
...
$em->persist($c);
$em->flush();
$response = new Response($this->getSerializer()->serialize($c, 'json'));
$response->headers->set('Content-Type', 'application/json');
return $response;
} else {
$response = array('code' => 'invalid', 'details' => $this->getErrorMessages($form));
return new JsonResponse($response, 400);
}
}
class C
{
/**
* #ORM\OneToMany(targetEntity="A", mappedBy="c", cascade={"persist", "remove"}, indexBy="id")
* #Assert\Valid()
* #Serializer\Expose
*/
protected $a;
A's relationship to C
/**
* #var \C
*
* #ORM\ManyToOne(targetEntity="C", inversedBy="a")
* #ORM\JoinColumns({
* #ORM\JoinColumn(name="c_id", referencedColumnName="id")
* })
* #Assert\NotBlank
*/
private $c;
And CType
class CType extends AbstractType
{
$builder->add(
'a',
'collection',
array(
'type' => new AType(),
'by_reference' => false,
'allow_add' => true,
'allow_delete' => true
)
);