Symfony2：init EntityType字段，包含空数据

Question

I have a field (EntityType => select in the view) in my FormBuilder that I want it to be in initialized with empty data so I can fill it after that in the view via ajax.
So I have read symfony's documentation about EntityType and I found the choices attribute that receives an array of data, so I gave it an empty one 'choices' => array() and it did the trick.
Now the problem is when I submit the form, symfony don't know anymore the type of the field and give me null.
This is the builder:

$buidler->add('supplier', EntityType::class, array(
                'class' => 'SBC\TiersBundle\Entity\Supplier',
                'attr' => array(
                    'class' => 'uk-select uk-select-supplier'
                ),
                'choices' => array(),
            ))

As you can see the the type of the field is SBC\TiersBundle\Entity\Supplier but after submit symfony gives me null !
What should I do to achieve my goal?

Answer 1

All right, this is the solution:
First, I need to pass the EntityManager to my form, and to do this I have created a service:

services:
    payment.supplier.form:
        class: SBC\PaymentBundle\Form\PaymentSupplierType
        tags:
            - { name: form.type, alias: form_em }
        arguments: ["@doctrine.orm.entity_manager"]

Then call the EntityManager in the __construct function:

 private $em;
 private $supplier;

 function __construct(EntityManager $em)
 {
    $this->em = $em;
 }

Second, I need to add two events to the form:
PRE_SUBMIT (to get supplier's code and create Supplier object using the EntityManager):

$builder->addEventListener(
            FormEvents::PRE_SUBMIT,
            function(FormEvent $event){
                $data = $event->getData();
                $code = $data['supplier'];
                $this->supplier = $this->em->getRepository('TiersBundle:Supplier')->find($code);
            }
        );

And finally, use the POST_SUBMIT event to set the supplier object in the submitted data:

$builder->addEventListener(
            FormEvents::POST_SUBMIT,
            function(FormEvent $event){
                $object = $event->getData();
                $object->setSupplier($this->supplier);
                $event->setData($object);

            }
        );

Thanx to Виталий Бойко who gave me a hint about the form events.
So this is what I did with my knowledge and if you have a better solution please share it with us.