symfony2集合选择实体

Question

有这个......

$builder->add('city', 'entity', array(
                'class' => '..\Entity\City',
                'translation_domain' => $this->translation_domain,
                'required' => true,
                'expanded' => true,
                'invalid_message' => 'form.error.city',
                'empty_value' => 'form.placeholder.city',
                'query_builder' => function (...\Repository\CityRepository $repository) {
                    return $repository->get(array(
                        'enabled' => true,
                        'locale' => $this->container->get('poisk_raboty.helper.main')->getLocale(),
                    ), true);
                },
                'attr' => array(
                    'data-search' => 2,
                    'data-placeholder' => $this->container->get('translator')->trans('form.placeholder.city', array(), $this->translation_domain),
                ),
            ))
        ;

如何在表单构建器中获取城市，其中一个元素必须像城市一样？