symfony security related problem

Hello, I have that a user can have one or several certificates and a certificate only belongs to one user.

this is the users entity

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Security\Core\User\UserInterface;

/**
 * @ORM\Table(name="app_users")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\UserRepository")
 */
class User implements UserInterface, \Serializable
{

     /**
     * @ORM\Column(type="integer")
     */
    private $idcertificado;


    /**
     * @ORM\ManyToOne(targetEntity="Certificado", inversedBy="users")
     * @ORM\JoinColumn(name="idcertificado", referencedColumnName="id")
     */
    private $certificado;

....

And the certified entity

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * certificado
 *
 * @ORM\Table(name="certificado")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\CertificadoRepository")
 */
class certificado
{

    /**
     * @ORM\OneToMany(targetEntity="User", mappedBy="certificado")
     */
    private $users;

    public function __construct()
    {
        $this->users = new ArrayCollection();
    }

....

The Controller to create a new user is

public function newAction(Request $request)
{
    $user = new User();
    $form = $this->createForm('AppBundle\Form\UserType', $user);
    $form->handleRequest($request);

    if ($form->isSubmitted() && $form->isValid()) {
        $encoder = $this->container->get('security.password_encoder');
        $encoded = $encoder->encodePassword($user, $user->getPassword());
        $user->setPassword($encoded);
        $user->setUsername($user->getEmail());
        $user->setRol('admin'');//Este solo es el rol de usuario
        $em = $this->getDoctrine()->getManager();
        $em->persist($user);
        $em->flush($user);

        return $this->redirectToRoute('user_show', array('id' => $user->getId()));
    }

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

The form is

namespace AppBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\Extension\Core\Type\PasswordType;
use Symfony\Component\Form\Extension\Core\Type\EmailType;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;

class UserType extends AbstractType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('username')
        ->add('password',PasswordType::class)
        ->add('email',EmailType::class)
        ->add('isActive')
        ->add('idcertificado',ChoiceType::class, array(
    'choices'  => array(
        'Seleccione el rol' => null,
        'Basico' => 1,
        'Avanzado' => 2,
                       )));
    }

When saving the idcertificate it comes null. I do not understand why. This only happens to me with this. With the relationship of a normal entity this does not happen to me.