symfony form entity update

I'm trying to create a for with an entity.

My controller:

$questionnaire = $em->getRepository('questionnaireBundle:questionnaire')->findOneBy(array('id' => $id));

        $form = $this->createForm(new questionnaireType(), $questionnaire);

QuestionnaireType:

public function buildForm(FormBuilderInterface $builder, array $options) {
    $builder->add('nom', 'text', array('label' => 'Nom:'));
    $builder->add('nbreQuestions', 'text', array('label' => 'Nombre de questions:'));
    $builder->add('type', 'entity', array('class' => 'questionnaireBundle:type', 'property' => 'type'));
    $builder->add('envoyer', 'submit', array('label' => 'Enregistrer', 'attr' => array('class' => 'btn btn-success col-md-12')));
}

The entity: questionnaire

namespace questionnaireBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * questionnaire
 *
 * @ORM\Table()
 * @ORM\Entity
 */
class questionnaire {

    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

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

    /**
     * @var integer
     *
     * @ORM\Column(name="nbreQuestions", type="integer")
     *
     * @Assert\NotBlank(message="Veuillez entrer un nombre de questions.")
     * @Assert\Type(type="integer")
     */
    private $nbreQuestions;

    /**
     * Get id
     *
     * @return integer 
     */
    public function getId() {
        return $this->id;
    }

    /**
     * Set nom
     *
     * @param string $nom
     * @return questionnaire
     */
    public function setNom($nom) {
        $this->nom = $nom;

        return $this;
    }

    /**
     * Get nom
     *
     * @return string 
     */
    public function getNom() {
        return $this->nom;
    }
    /**
     * Set type
     *
     * @param string $type
     * @return type
     */
    public function setType($type) {
        $this->type = $type;

        return $this;
    }

    /**
     * Get type
     *
     * @return string 
     */
    public function getType() {
        return $this->type;
    }

    /**
     * Set nbreQuestions
     *
     * @param integer $nbreQuestions
     * @return questionnaire
     */
    public function setNbreQuestions($nbreQuestions) {
        $this->nbreQuestions = $nbreQuestions;

        return $this;
    }

    /**
     * Get nbreQuestions
     *
     * @return integer 
     */
    public function getNbreQuestions() {
        return $this->nbreQuestions;
    }

    public function __toString() {
        return (string) $this->getNom();
    }

}

The field nom, nbreQuestions will automatically be fully with the entity questionnaire, but type is not update!

Anybody knows why?

Thanks Best regards