Constraints\Callback annotation validate the previous (not submitted) entity - forms

Question

I'm using a Callback to validate an entity
/**
* #ORM\Entity(repositoryClass="AppBundle\Repository\Foo")
* #ORM\Table(name="foo")
* #Constraints\Callback(methods={"validate"})
*/
class Foo
{
...
function validate(ExecutionContextInterface $context)
{
if ($this->foos) {
$context->buildViolation('Foos cannot be emty')
->atPath('foos')
->addViolation();
}
}
A form is using this entity:
class FooFormType extends AbstractType
{
private $foosService;
function __construct(FoosService $foosService) {
$this->foosService = $foosService;
}
/**
* #param FormBuilderInterface $builder
* #param array $options
*/
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('foos', EntityType::class, array(
'label' => false,
'class' => 'AppBundle:Entity\Foo',
'choices' => $this->foosService->getSomeFoos($builder->getData()),
'expanded' => true,
'multiple' => true,
'required' => true,
))
->getForm();
}
/**
* #param OptionsResolver $resolver
*/
public function configureOptions(OptionsResolver $resolver)
{
$resolver->setDefaults(array(
'data_class' => 'AppBundle\Entity\Foo',
));
}
}
This form is called within a factory:
public function getFooForm(Foo $foo)
{
return $this->formFactory->create(new FooFormType($this->foosService), $foo);
}
Finally, in the controller:
...
$form = $this->get('my_factory')->getFooForm($foo);
$form->handleRequest($request);
if ($form->isSubmitted() && $form->isValid()) {
$this->getDoctrine()->getManager()->flush();
...
}
...
This seems to work correctly, but the validation is being done over the previous entity. I mean, if I submit empty foos I can see the validation error, but if I sumbit some foos, and then I remove those foosand submit again, the validation doesn't throw any exception because it's being done over the last entity, which had some foos. I've checked that the submitted data is correct, and indeed, Foo entity is been persisted with empty foos (only if the previous one had foos).
What can be causing this weird behaviour?