Unable to guess how to get a Doctrine instance from the request inform

tout ira bien avant que j'ajoute ma fonction editer() et donc ce message apparait

public function editerAction(Article $article)
    {
     $em = $this->getDoctrine()->getManager();
     $form = $this->createForm(new ArticleType(),$article);
     $request = $this->getRequest();
     if($request->isMethod('POST')){
        $form->bindRequest($request);
        if($form->isValid()){
            $a = $form->getData();
            $em->persist($a);
            $em->flush();
            return $this->redirect(
                $this->generateUrl('yoss_admin_bundle_article',array(
                    'id' =>$a->getId(),
                    ))
                );
              }
            }
       return $this->render('YossAdminBundle:Admin:editer.html.twig',array(
        'form' =>createForm(),
        ));

    }