problème de form

Bonjour,

Voila je rencontre un petit problème avec mon code.

Ce que je fais

Dans la page show_message, il y a un form qui permet de laisser une message, avec les fields "title, message, parents_id"

    namespace Yoda\MessageBundle\Entity;
    ...
  /**
     * @ORM\ManyToOne(targetEntity="Message")
     * @ORM\JoinColumn(name="parents_id", referencedColumnName="id")
     */
     public $parents;
    function getParents()
    {
        return $this->parents;
    }

    function setParents(Message $parents)
    {
        $this->parents = $parents;
        return $this;
    }
    ...

    namespace Yoda\MessageBundle\Form;
    ...
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('title')
            ->add('message', TextareaType::class, array('attr' => array('class' => 'tinymce')))

            //->add('active')
            ->add('parents', HiddenType::class);
            //->add('owner');

    }
    ...

namespace Yoda\MessageBundle\Controller;
    public function newAction(Request $request)
    {
        $message = new Message();
        $form = $this->createForm('Yoda\MessageBundle\Form\MessageType', $message);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            $em = $this->getDoctrine()->getManager();
            $message->setCreateDate(new \DateTime());
            $message->setOwner($this->getUser());
            $em->persist($message);
            $em->flush();
            // if it is a children message, we will show parents message
            $redirectId =  $message->getParents()->getId() ? $message->getParents()->getId() : $message->getId();
            return $this->redirectToRoute('message_show', array('id' => $redirectId));
        }

        return array(
            'message' => $message,
            'form' => $form->createView(),
        );
    }

    public function showAction(Message $message)
    {
        $addForm = $this->createForm('Yoda\MessageBundle\Form\MessageType');
        $addForm->get('parents')->setData($message->getId());
        $childrenMessage = $this->getChildren($message);
        //var_dump($childrenMessage);
        return array(
            'add_form' => $addForm->createView(),
            'message' => $message,
            'childrens' => $childrenMessage,
        );        
    }

Ce que je veux

quand on répond, on ajoute une ligne dans la base.

Ce que j'obtiens

Quand on commit, Il me retourne une message érreur

Catchable Fatal Error: Argument 1 passed to Yoda\MessageBundle\Entity\Message::setParents() must be an instance of Yoda\MessageBundle\Entity\Message, string given, called in C:\xampp\htdocs\symfony_projet2\vendor\symfony\symfony\src\Symfony\Component\PropertyAccess\PropertyAccessor.php on line 556 and defined