Symfony FormEvent。必须管理传递到选择字段的实体

Question

我尝试使用EventListener但是我有错误： 必须管理传递到选择字段的实体。也许坚持他们在实体经理？

ExampleType.php

$builder->addEventListener(
            FormEvents::PRE_SET_DATA,
            function (FormEvent $event) {
                $form = $event->getForm();

                // this would be your entity, i.e. SportMeetup
                $data = $event->getData();

                $category = $data->getCategory();
                $tagi = null === $category ? array() : $category->getTags();

                $form->add('tags', 'entity', array(
                    'class'       => 'MyAppBundle:Tag',
                    'placeholder' => '',
                    'choices'     => $tagi,
                ));
            }
        );

// Category.php（entity）

/**
 * Get tags
 *
 * @return \Doctrine\Common\Collections\Collection
 */
public function getTags()
{
    return $this->tags;
}