symfony2 Erreur Fatale: un Objet de la classe n'a pas pu être converti en chaîne

J'ai cette Entité définie:

<?php
namespace Apw\BlackbullBundle\Entity;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;
/**
* Categories
*
* @ORM\Table()
*    @ORM\Entity(repositoryClass="Apw\BlackbullBundle\Entity\CategoriesRepository")
*/
class Categories
{
/**
* @var integer
*
* @ORM\Column(name="id", type="integer")
* @ORM\Id
* @ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* @var string
*
* @ORM\Column(name="categories_image", type="string", length=64, nullable = true)
*/
private $categoriesImage;
/**
* @var integer
*
* @ORM\Column(name="parent_id", type="integer", nullable = true, options={"default":0})
*/
private $parentId;
/**
* @var integer
*
* @ORM\Column(name="sort_order", type="integer", nullable = true, options={"default":0})
*/
private $sortOrder;
/**
* @var \DateTime
*
* @ORM\Column(name="date_added", type="datetime", nullable = true)
*/
private $dateAdded;
/**
* @var \DateTime
*
* @ORM\Column(name="last_modified", type="datetime", nullable = true)
*/
private $lastModified;
/**
* @var boolean
*
* @ORM\Column(name="categories_status", type="boolean", nullable = true, options={"default" = 1})
*/
private $categoriesStatus;
/**
* @ORM\OneToMany(targetEntity="CategoriesDescription", mappedBy="category", cascade={"persist", "remove"})
*/
private $categoryDescription;
/**
* @ORM\ManyToMany(targetEntity="Products", mappedBy="categories")
**/
private $products;
/**
* Get id
*
* @return integer
*/
public function getId()
{
return $this->id;
}
/**
* Set categoriesImage
*
* @param string $categoriesImage
* @return Categories
*/
public function setCategoriesImage($categoriesImage)
{
$this->categoriesImage = $categoriesImage;
return $this;
}
/**
* Get categoriesImage
*
* @return string
*/
public function getCategoriesImage()
{
return $this->categoriesImage;
}
/**
* Set parentId
*
* @param integer $parentId
* @return Categories
*/
public function setParentId($parentId)
{
$this->parentId = $parentId;
return $this;
}
/**
* Get parentId
*
* @return integer
*/
public function getParentId()
{
return $this->parentId;
}
/**
* Set sortOrder
*
* @param string $sortOrder
* @return Categories
*/
public function setSortOrder($sortOrder)
{
$this->sortOrder = $sortOrder;
return $this;
}
/**
* Get sortOrder
*
* @return string
*/
public function getSortOrder()
{
return $this->sortOrder;
}
/**
* Set dateAdded
*
* @param \DateTime $dateAdded
* @return Categories
*/
public function setDateAdded($dateAdded)
{
$this->dateAdded = $dateAdded;
return $this;
}
/**
* Get dateAdded
*
* @return \DateTime
*/
public function getDateAdded()
{
return $this->dateAdded;
}
/**
* Set lastModified
*
* @param \DateTime $lastModified
* @return Categories
*/
public function setLastModified($lastModified)
{
$this->lastModified = $lastModified;
return $this;
}
/**
* Get lastModified
*
* @return \DateTime
*/
public function getLastModified()
{
return $this->lastModified;
}
/**
* Constructor
*/
public function __construct()
{
$this->categoryDescription = new ArrayCollection();
$this->products = new ArrayCollection();
}
/**
* Add categoryDescription
*
* @param \Apw\BlackbullBundle\Entity\CategoriesDescription $categoryDescription
* @return Categories
*/
public function addCategoryDescription(\Apw\BlackbullBundle\Entity\CategoriesDescription $categoryDescription)
{
$this->categoryDescription[] = $categoryDescription;
return $this;
}
/**
* Remove categoryDescription
*
* @param \Apw\BlackbullBundle\Entity\CategoriesDescription $categoryDescription
*/
public function removeCategoryDescription(\Apw\BlackbullBundle\Entity\CategoriesDescription $categoryDescription)
{
$this->categoryDescription->removeElement($categoryDescription);
}
/**
* Get categoryDescription
*
* @return \Doctrine\Common\Collections\Collection
*/
public function getCategoryDescription()
{
return $this->categoryDescription;
}
/**
* Add products
*
* @param \Apw\BlackbullBundle\Entity\Products $products
* @return Categories
*/
public function addProduct(\Apw\BlackbullBundle\Entity\Products $products)
{
$this->products[] = $products;
return $this;
}
/**
* Remove products
*
* @param \Apw\BlackbullBundle\Entity\Products $products
*/
public function removeProduct(\Apw\BlackbullBundle\Entity\Products $products)
{
$this->products->removeElement($products);
}
/**
* Get products
*
* @return \Doctrine\Common\Collections\Collection
*/
public function getProducts()
{
return $this->products;
}
/**
* Set categoriesStatus
*
* @param boolean $categoriesStatus
* @return Categories
*/
public function setCategoriesStatus($categoriesStatus)
{
$this->categoriesStatus = $categoriesStatus;
return $this;
}
/**
* Get categoriesStatus
*
* @return boolean
*/
public function getCategoriesStatus()
{
return $this->categoriesStatus;
}
}

Puis j'ai cette méthode dans mon contrôleur pour gérer l'envoi du formulaire:

<?php
namespace Apw\BlackbullBundle\Controller;
use Apw\BlackbullBundle\Entity\Categories;
use Apw\BlackbullBundle\Entity\CategoriesDescription;
use Apw\BlackbullBundle\Form\CategoriesType;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Template;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\Form\Extension\Core\ChoiceList\ChoiceList;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Security;
class CategoriesController extends Controller
{
/**
* @Security("has_role('ROLE_ADMIN')")
* @Route("/createCategory")
* @Template()
*/
public function createCategoryAction(Request $request){
$category = new Categories();
$categoryDesc = new CategoriesDescription();
$category->addCategoryDescription($categoryDesc);
$categoryDesc->setCategory($category);
$form = $this->createForm(new CategoriesType(), $category);
$form->handleRequest($request);
if($form->isValid()){
//exit(\Doctrine\Common\Util\Debug::dump($category));
$em = $this->getDoctrine()->getManager();
$em->persist($category);
$em->persist($categoryDesc);
$em->flush();
return $this->redirect($this->generateUrl('apw_blackbull_categories_showcategories'));
}
return array(
'form' => $form->createView()
);
}
}

Et enfin, c'est ma CategoryType.php:

<?php
namespace Apw\BlackbullBundle\Form;
use Doctrine\ORM\EntityRepository;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
class CategoriesType extends AbstractType
{
/**
* @param FormBuilderInterface $builder
* @param array $options
*/
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('categoryDescription', 'collection',
array(
'type' => new CategoriesDescriptionType(),
'allow_add' => true,
'options' => array('data_class' => 'Apw\BlackbullBundle\Entity\CategoriesDescription'),
'by_reference' => false,
))
->add('categoriesImage', null, array('label'=>'Foto:'))
->add('categoriesStatus', null, array('label'=>'Stato:'))
->add('parentId', 'entity', array( //provare a mettere una querybuiler
'class'       => 'ApwBlackbullBundle:CategoriesDescription',
'property'    => 'categoriesName',
'empty_value' => 'Scegliere una categoria',
'required'    => false,
'label'       => 'Crea in:'))
->add('salva','submit')
->add('azzera','reset')
;
}
/**
* @param OptionsResolverInterface $resolver
*/
public function setDefaultOptions(OptionsResolverInterface $resolver)
{
$resolver->setDefaults(array(
'data_class' => 'Apw\BlackbullBundle\Entity\Categories',
));
}
/**
* @return string
*/
public function getName()
{
return 'categories';
}
}

Quand j'essaie d'enregistrer les données, j'obtiens cette erreur:

Une exception s'est produite lors de l'exécution 'INSÉRER DANS les Catégories
(categories_image, parent_id, sort_order, date_added, last_modified,
categories_status) VALEURS (?, ?, ?, ?, ?, ?)' avec params ["tel", {},
null, null, null, 1]:

Erreur Fatale: un Objet de la classe
Apw\BlackbullBundle\Entity\CategoriesDescription n'a pas pu être
converti en chaîne

Ce que je fais mal?