Validating dynamically loaded choices in Symfony 2

Question

I have a choice field type named *sub_choice* in my form whose choices will be dynamically loaded through AJAX depending on the selected value of the parent choice field, na

Answer 1

this accept any value

 $builder->addEventListener(FormEvents::PRE_SUBMIT, function (FormEvent $event) {
    $data = $event->getData();
    if(is_array($data['tags']))$data=array_flip($data['tags']);
    else $data = array();
    $event->getForm()->add('tags', 'tag', [
        'label'   => 'Sub Choice',
        'choices' => $data,
        'mapped'=>false,
        'required'=>false,
        'multiple'=>true,
    ]);
});