Symfony2表单:如何持久化具有可空连接的实体？

/**
 * @ORM\ManyToOne(targetEntity="RelatedEntity")
 * @ORM\JoinColumn(nullable=true)
 */
private $relatedEntity;

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        ->add('relatedEntity', EntityType::class, array(
            'class' => 'AppBundle:RelatedEntity',
            'choice_label' => 'name',
            'required' => false,
            'placeholder' => '-- 请选择 --',
        ));
}

php bin/console doctrine:schema:update --force