Symfony2 get object from inverted entity with ManyToMany relation

I'm facing to a problem with my Doctrine entities relations. Here's the matter : I have 2 entities : Article and Category Article is the master, category is the slave I'd like to get Categories from Articles and the other way, Articles from Categories. I made a ManyToMany relation like this : ``` class Article { /** * @ORM\ManyToMany(targetEntity="Alpha\BlogBundle\Entity\Category", cascade={"persist"}, inversedBy="Article") * @ORM\JoinTable(name="article_category") */ private $categories; ``` and ``` public function __construct(){ $this->categories = new \Doctrine\Common\Collections\ArrayCollection(); ``` and the in Category entity : ``` class Category { /** * @ORM\ManyToMany(targetEntity="Alpha\BlogBundle\Entity\Article", cascade={"persist"}, mappedBy="Category") */ private $articles; ``` and ``` public function __construct(){ $this->articles = new \Doctrine\Common\Collections\ArrayCollection(); ``` In my article entity, I also added the folowing : ``` public function addCategory(\Alpha\BlogBundle\Entity\Category $categories) { $this->categories[] = $categories; $categories->addArticle($this); return $this; } ``` (the forth line, `$categories->addArticle($this);`) In my controller : ``` public function ajouterAction($data = null, $id = null) { // On récupère l'EM pour enregistrer en BDD $em = $this->getDoctrine()->getManager(); // On définit une nouvel objet Article avec de nouveaux attributs $article = new Article; $article->setTitle('1er article !'); $article->setContent('Cupcake ipsum dolor sit amet ice cream tiramisu unerdwear.com. Caramels halvah lollipop apple pie soufflé. Tart lollipop soufflé candy tootsie roll sweet donut. Lemon drops danish I love icing I love. Candy canes cheesecake I love. I love tiramisu applicake. I love gingerbread soufflé sweet roll muffin. Cupcake liquorice gummi bears muffin chocolate jelly-o.'); $article->setAuthor('Toto'); // On définit une nouvel objet Category avec de nouveaux attributs $category = new Category; $category->setName('Poney'); $article->addCategory($category); $em->persist($category); $em->persist($article); $em->flush(); return $this->render('AlphaBlogBundle:Blog:ajouter.html.twig'); } ``` And to finish, to get my articles from the categories : ``` public function categoryAction($cat = null) { $em = $this->getDoctrine()->getManager(); // Si cat est vide, on renvoit la liste complète des catégories if (!isset($cat) || empty($cat) || $cat == null) { $categories = $em->getRepository('AlphaBlogBundle:Category')->findAll(); return $this->render('AlphaBlogBundle:Blog:categories.html.twig', array( 'categories' => $categories )); } // Sinon on renvoit la liste des articles de la catégorie else { $category = $em->getRepository('AlphaBlogBundle:Category')->findOneBy(array('name' => $cat)); $articles = $category->getArticles(); return $this->render('AlphaBlogBundle:Blog:category.html.twig', array( 'articles' => $articles, 'category' => $category //'name' => $name )); } } ``` In my view, I can see the name of my category, but the article is not showing, I have this error message : `ContextErrorException: Notice: Undefined index: Category in /home/franck/www/alpha/vendor/doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php line 1036` I'm a bit lost here, if someone could help.