symfony2 don't validate file upload on update - symfony

Question

I want to update the single entity field using ajax. Basically I don't have form, Simply I am triggering ajax by clicking link passing id and value. But I have mutiple file fields in entity form. So while I update the entity PrePersist and PostPersist functions are triggering for file upload. I don't want to do this on this update.
My Controller Action
public function ajaxupdateAction(Request $request){
$data = $request->query->get('data');
$id = $data['id'];
$em = $this->getDoctrine()->getManager();
$entry = $em->getRepository('RootContestBundle:Entry')->find($id);
if (!$entry) {
throw $this->createNotFoundException('Unable to find Entry entity.');
}
$form = $this->createFormBuilder(array('id' => $id,'is_liked'=>true))
->add('id', 'hidden')
->add('is_liked','hidden')
->getForm();
$entry->setIsLiked(true);
$form->bind($this->getRequest());
$em->persist($entry);
$em->flush();
return new JsonResponse(array('reverse'=>'dislike'));
}
What I am doing wrong, How can I solve this !

Answer 1

Here you got an documentation about the form event subscribers:
http://symfony.com/doc/2.0/cookbook/form/dynamic_form_generation.html
Use the postBind event.
Then check if the form isValid()
If it is valid call your service responsible for file upload.
Here is how I did this:
public function postBind(FormEvent $event)
{
$data = $event->getData();
$form = $event->getForm();
$file = $form->get('upload')->getData();
if($file && $form->isValid())
{
$result = $this->upload->uploadFile($file);
$data->setUpload($result);
}
$event->setData($data);
}
My method has the upload service injected by DI and assigned to variable $this->upload.