Symfony 2 | Form exception when modifying an object that has a file(picture) field

Question

I'm using Symfony2. I have an entity Post that has a title and a picture field.

My problem : Everything is fine when I create a post, I have my picture etc. But when I want to modify it, I have a problem with the "picture" field which is an uploaded file, Symfony wants a file type and it has a string (the path of the uploaded file) :

The form's view data is expected to be an instance of class Symfony\Component\HttpFoundation\File\File, but is a(n) string. You can avoid this error by setting the "data_class" option to null or by adding a view transformer that transforms a(n) string to an instance of Symfony\Component\HttpFoundation\File\File. 

I'm really stuck with this problem and really don't know how to solve it, any help would be greatly appreciated! Thanks a lot!

Here is my PostType.php (which is used in newAction() and modifiyAction()) and which may cause the problem (Form/PostType.php) :

<?php
namespace MyBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\HttpFoundation\File\UploadedFile;

use MyBundle\Entity\Post;

class PostType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
        ->add('title')
        ->add('picture', 'file');//there is a problem here when I call the modifyAction() that calls the PostType file.
    }

    public function getDefaultOptions(array $options)
    {
        return array(
            'data_class' => 'MyBundle\Entity\Post',
        );
    }

    public static function processImage(UploadedFile $uploaded_file, Post $post)
    {
        $path = 'pictures/blog/';
        //getClientOriginalName() => Returns the original file name.
        $uploaded_file_info = pathinfo($uploaded_file->getClientOriginalName());
        $file_name =
            "post_" .
            $post->getTitle() .
            "." .
            $uploaded_file_info['extension']
            ;

        $uploaded_file->move($path, $file_name);

        return $file_name;
    }

    public function getName()
    {
        return 'form_post';
    }
}

Here is my Post entity (Entity/Post.php) :

<?php

namespace MyBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

use Symfony\Component\Validator\Constraints as Assert;

/**
 * MyBundle\Entity\Post
 *
 * @ORM\Table()
 * @ORM\Entity
 */
class Post
{
    /**
     * @var integer $id
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @ORM\Column(type="string", length=255, nullable=true)
     * @Assert\Image(
     *      mimeTypesMessage = "Not valid.",
     *      maxSize = "5M",
     *      maxSizeMessage = "Too big."
     *      )
     */
    private $picture;

    /**
     * @var string $title
     *
     * @ORM\Column(name="title", type="string", length=255)
     */
    private $title;

   //getters and setters
   }

Here is my newAction() (Controller/PostController.php) Every works fine with this function:

public function newAction()
{
    $em = $this->getDoctrine()->getEntityManager();
    $post = new Post();
    $form = $this->createForm(new PostType, $post);
    $post->setPicture("");
    $form->setData($post);
    if ($this->getRequest()->getMethod() == 'POST') 
    {
        $form->bindRequest($this->getRequest(), $post);
        if ($form->isValid()) 
        {
            $uploaded_file = $form['picture']->getData();
            if ($uploaded_file) 
            {
                $picture = PostType::processImage($uploaded_file, $post);
                $post->setPicture('pictures/blog/' . $picture);
            }
            $em->persist($post);
            $em->flush();
            $this->get('session')->setFlash('succes', 'Post added.');

            return $this->redirect($this->generateUrl('MyBundle_post_show', array('id' => $post->getId())));
        }
    }

    return $this->render('MyBundle:Post:new.html.twig', array('form' => $form->createView()));
}

Here is my modifyAction() (Controller/PostController.php) :There is a problem with this function

public function modifyAction($id)
{
    $em = $this->getDoctrine()->getEntityManager();
    $post = $em->getRepository('MyBundle:Post')->find($id);
    $form = $this->createForm(new PostType, $post);//THIS LINE CAUSES THE EXCEPTION
    if ($this->getRequest()->getMethod() == 'POST') 
    {
        $form->bindRequest($this->getRequest(), $post);
        if ($form->isValid()) 
        {
            $uploaded_file = $form['picture']->getData();
            if ($uploaded_file) 
            {
                $picture = PostType::processImage($uploaded_file, $post);
                $post->setPicture('pictures/blog/' . $picture);
            }
            $em->persist($post);
            $em->flush();
            $this->get('session')->setFlash('succes', 'Modifications saved.');

            return $this->redirect($this->generateUrl('MyBundle_post_show', array('id' => $post->getId())));
        }
    }
    return $this->render('MyBundle:Post:modify.html.twig', array('form' => $form->createView(), 'post' => $post));
}

Answer 1

I solved the problem setting data_class to null as follows:

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
    ->add('title')
    ->add('picture', 'file', array('data_class' => null)
    );
}