创建工作正常，但更新失败时，使用完全相同的实体和形式

下面的例子工作正常，当我尝试创建一个新的car在数据库中。如果我在webform中留下brands选择框空白，我在提交后得到"品牌字段是必需的"错误，这是完全好的，正如预期的。

问题:

如果我在尝试更新所选记录时重复上述完全相同的步骤，我应该在提交后仍然得到"Brand field is required"错误，而不是得到"ContextErrorException: Catchable Fatal Error: Argument 1 passed to Car'BrandBundle'Entity'Cars::setBrands() must be an instance of Car'BrandBundle'Entity'Brands, null given, called in .... line on public function setBrands('Car'BrandBundle'Entity'Brands $brands)"

有什么原因或解决办法吗?

CAR UPDATE CONTROLLER (DOESN'T WORK)

private function getForm($car, $id)
{
    return $this->createForm(new CarsType(), $car,
        array('action' => $this->generateUrl('cars_independent_update_process',
            array('id' => $id))));
}
public function processAction(Request $request, $id)
{
    $repo = $this->getDoctrine()->getRepository('CarBrandBundle:Cars');
    $car = $repo->findOneBy(array('id' => $id));
    if (! $car) 
    {
        return new Response('There is no such car in database');
    }
    $form = $this->getForm($car, $id);
    $form->handleRequest($request);
    if ($form->isValid() !== true)
    {
        return $this->render('CarBrandBundle:Independent:cars_update.html.twig',
            array('page' => 'Cars Update Independent', 'form' => $form->createView()));
    }
    exit('FINE');
}

CAR CREATE CONTROLLER (WORKS FINE)

class CarsCreateController extends Controller
{
    private function getForm()
    {
        return $this->createForm(new CarsType(), new Cars(),
                array('action' => $this->generateUrl('cars_independent_create_process')));
    }
    public function processAction(Request $request)
    {
        $form = $this->getForm();
        $form->handleRequest($request);
        if ($form->isValid() !== true)
        {
            return $this->render('CarBrandBundle:Independent:cars_update.html.twig',
            array('page' => 'Cars Update Independent', 'form' => $form->createView()));
        }
    exit('FINE');
}

实体

class Cars
{
    /**
     * @ORM'ManyToOne(targetEntity="Brands", inversedBy="cars")
     * @ORM'JoinColumn(name="brands_id", referencedColumnName="id", nullable=false)
     * @Assert'NotBlank(message="The Brand field is required.")
     */
    protected $brands;
    /**
     * @param 'Car'BrandBundle'Entity'Brands $brands
     */
    public function setBrands('Car'BrandBundle'Entity'Brands $brands)
    {
        $this->brands = $brands;
        return $this;
    }
    /**
     * @return 'Car'BrandBundle'Entity'Brands
     */
    public function getBrands()
    {
        return $this->brands;
    }
}

表单类型

class CarsType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->setAction($options['action'])
            ->setMethod('POST')
            ->add('brands', 'entity',
                array(
                    'error_bubbling' => true,
                    'class' => 'CarBrandBundle:Brands',
                    'property' => 'name',
                    'multiple' => false,
                    'expanded' => false,
                    'empty_value' => '',
                    'query_builder' => function (EntityRepository $repo)
                                        {
                                            return $repo->createQueryBuilder('b')
                                                        ->orderBy('b.name', 'ASC');
                                        }
                ))
            ->add('model', 'text', array('label' => 'Model', 'error_bubbling' => true))
            ->add('year', 'date', array('label' => 'Year', 'error_bubbling' => true))
            ->add('button', 'submit', array('label' => 'Submit'))
        ;
    }
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array('data_class' => 'Car'BrandBundle'Entity'Cars'));
    }
    public function getName()
    {
        return 'cars';
    }
}