'NotNullConstraintViolationException' Error when flush() on Symfony 3

I'm starting to learn Symfony 3, but i encountered an error when trying to flush an Objet on my Database using Doctrine.

On my main Controller, I instantiate an Objet on which I set some properties default values.

<?php
namespace AppBundle\Controller;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use AppBundle\Entity\Objet;
use AppBundle\Entity\Membre;

class DefaultController extends Controller
{
    /**
    * @Route("/new", name="nouvelobjet")
    */
    public function newAction(Request $request)
    {
        $objet = new Objet();
        $objet->setIdVendeur('2');
        $objet->setTitre('Article de test');
        $em = $this->getDoctrine()->getManager();
        $em->persist($objet);
        $em->flush();
        return new Response ('<body>Id_vendeur:'.$objet->getIdVendeur().'</body>');
    }
} 
?>

Here is the Classe 'Objet' I use :

<?php
// src/AppBundle/Entity/Objet.php
namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
/**
 * @ORM\Entity
 * @ORM\Table(name="objet")
 */

class Objet
{
    /**
     * @ORM\Column(type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id_article;

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

    /**
     * @ORM\Column(type="string")
     */
    private $titre;

    /**
     * @ORM\ManyToOne(targetEntity="Membre", inversedBy="objets")
     * @ORM\JoinColumn(name="id_vendeur", referencedColumnName="id")
     */
    private $membre;

    /**
     * Get idArticle
     *
     * @return integer
     */
    public function getIdArticle()
    {
        return $this->id_article;
    }

    /**
     * Set idVendeur
     *
     * @param integer $idVendeur
     *
     * @return Objet
     */
    public function setIdVendeur($idVendeur)
    {
        $this->id_vendeur = $idVendeur;

        return $this;
    }

    /**
     * Get idVendeur
     *
     * @return integer
     */
    public function getIdVendeur()
    {
        return $this->id_vendeur;
    }

    /**
     * Set titre
     *
     * @param string $titre
     *
     * @return Objet
     */
    public function setTitre($titre)
    {
        $this->titre = $titre;

        return $this;
    }

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


    /**
     * Set membre
     *
     * @param \AppBundle\Entity\Membre $membre
     *
     * @return Objet
     */
    public function setMembre(\AppBundle\Entity\Membre $membre = null)
    {
        $this->membre = $membre;

        return $this;
    }

    /**
     * Get membre
     *
     * @return \AppBundle\Entity\Membre
     */
    public function getMembre()
    {
        return $this->membre;
    }
}

and this is the class 'Membre' (not sure it's really useful there):

<?php
// src/AppBundle/Entity/Membre.php
namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
/**
 * Membre
 *
 * @ORM\Table(name="membre")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\MembreRepository")
 */
class Membre
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

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

    /**
     * @ORM\OneToMany(targetEntity="Objet", mappedBy="membre")
     */
    private $objets;

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

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

    /**
     * Set name
     *
     * @param string $name
     *
     * @return Membre
     */
    public function setName($name)
    {
        $this->name = $name;

        return $this;
    }

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

    /**
     * Add objet
     *
     * @param \AppBundle\Entity\Objet $objet
     *
     * @return Membre
     */
    public function addObjet(\AppBundle\Entity\Objet $objet)
    {
        $this->objets[] = $objet;

        return $this;
    }

    /**
     * Remove objet
     *
     * @param \AppBundle\Entity\Objet $objet
     */
    public function removeObjet(\AppBundle\Entity\Objet $objet)
    {
        $this->objets->removeElement($objet);
    }

    /**
     * Get objets
     *
     * @return \Doctrine\Common\Collections\Collection
     */
    public function getObjets()
    {
        return $this->objets;
    }
}

When I go the url of my controller, i throw an error :

An exception occurred while executing 'INSERT INTO objet (id_vendeur, titre) VALUES (?, ?)' with params [null, "Article de test"]: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'id_vendeur' cannot be null

When I comment the '$em->flush();' line, I can print the value of getIdVendeur() without problem, so it's not a null value.

It makes 3 days i'm stuck on this problem... can someone help me about this ? thanks in advance !

Solution

Assuming that in the database object Membre with id = 2 exist you can do the following in your controller:

public function newAction(Request $request)
    {
        $objet = new Objet();
        $objet->setIdVendeur('2');
        $objet->setTitre('Article de test');
        $em = $this->getDoctrine()->getManager();

        // Retreive the object instance with id = 2
        $membre = $em->find(Membre::class, 2);
        // Set/specify the doctrine relations  
        $objet->setMembre($membre);   

        $em->persist($objet);
        $em->flush();
        return new Response ('<body>Id_vendeur:'.$objet->getIdVendeur().'</body>');
    }

Probably you don't need the attribute id_vendeur of the class Objet.

Hope this help