Symfony ajax form validation

gistfile1.phtml

<?php


 * @Route("/{id}/edit", name="user_edit")
 * @Template()
 */
public function editAction($id)
{
    /** @var $dm \Doctrine\ODM\MongoDB\DocumentManager */
    $dm = $this->get('doctrine_mongodb')->getManager();

    /** @var $repository  */
    $repository = $dm->getRepository('UserBundle:User');

    $user = $repository->find($id);

    $form = $this->createFormBuilder($user, [
            'validation_groups' => ['settings'],
            'attr' => ['novalidate' => 'novalidate']
        ])
        ->add('firstName', 'text', ['required' => false])
        ->add('lastName', 'text')
        ->add('captcha', 'captcha')
        ->add('save', 'submit')
        ->getForm()
    ;

    $request = $this->getRequest();

    $form->handleRequest($request);

    if ($form->isValid()) {
        $dm->persist($user);
        $dm->flush();

        $url = $this->generateUrl('user_edit', ['id' => $id]);
        if ($request->isXmlHttpRequest()) {
            return new JsonResponse(['redirect' => $url]);
        }
        return $this->redirect($url);
    }
    else {
        if ($request->isXmlHttpRequest()) {
            $captcha = $form->get('captcha')->createView();

            return new JsonResponse(['img' => $captcha->vars['captcha_code']], 400);
        }
    }

    $view = $form->createView();
    return ['form' => $view];
}