Symfony2 Forms conditional validation on empty fields - php

Question

I've created a form which has some fields, whose required status is dependend on the value entered into another field (to be specific, whether the value is > 0).
It is about ordering a textbook and if the quantity is > 0, then the address-related fields must also be entered.
I've added a custom validation callback, which can check whether the count is > 0 and then adds a Violation. But it gets only executed if the field is not empty (since it is not "required").
Is there a way to always validate, but without having to use the required flag on the field?
Below a snippet:
[...]
$builder->add('firstname', 'text', array(
'label' => 'label.firstname',
'required' => false,
'property_path' => 'Order.firstname',
'constraints' => array(
new Callback(array('callback' => array($this, 'validateOrder')))
)
));
[...]
public function validateOrder($data, ExecutionContextInterface $context)
{
/** #var \Symfony\Component\Form\Form $form */
$form = $context->getRoot();
if(
(
(int)$form->get('count_d')->getData() > 0 ||
(int)$form->get('count_f')->getData() > 0 ||
(int)$form->get('count_i')->getData() > 0
) && $data == '') {
$context->addViolation('profile.order.error');
}
}

Answer 1

Wouldn't it be more appropriate to put callback at form directly (instead of field)?
public function setDefaultOptions(OptionsResolverInterface $resolver)
{
$resolver->setDefaults(array(
'constraints' => array(
new Callback(array('callback' => array($this, 'validateOrder')))
)
));
}
This way, the callback will be invoked in any case...