问Symfony2和Selectize.js:在实体字段类型中持久化新项的最清晰的方法？

EN

namespace AppBundle\Form;

use AppBundle\Entity\Tag;
use Doctrine\Common\Persistence\ObjectManager;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\OptionsResolver\OptionsResolver;

class PostType extends AbstractType
{
    /**
     * @var ObjectManager
     */
    private $manager;

    /**
     * Constructor
     *
     * @param ObjectManager $manager
     */
    public function __construct(ObjectManager $manager)
    {
        $this->manager = $manager;
    }

    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('title')
            ->add('content')
            ->add('tags')
        ;
        $builder->get('tags')->addEventListener(
            FormEvents::PRE_SUBMIT,
            function (FormEvent $event) {
                $choiceList = $event->getForm()->getConfig()->getAttribute('choice_list');
                $array = is_null($event->getData()) ? [] : $event->getData();
                $choices = $choiceList->getChoicesForValues($array);

                if (count($choices) !== count($array)) {
                    $values = $choiceList->getValuesForChoices($choices);
                    $diff = array_merge(array_diff($values, $array), array_diff($array, $values));

                    foreach ($diff as $value) {
                        $new = new Tag($value);
                        $this->manager->persist($new);
                        $this->manager->flush();
                        $values[] = $new->getId();
                    }

                    $event->setData($values);
                }
            }
        );
    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Entity\Post'
        ));
    }
}

class SubjectTransformer implements DataTransformerInterface
{
    protected $em;

    public function __construct($em)
    {
        $this->em = $em;
    }

    //public function transform($val) { ... }

    public function reverseTransform($str)
    {
        $repo = $this->em->getRepository('AppBundle:Subject');

        $subject = $repo->findOneByName($str);
        if($subject)
            return $subject;

        //Didn't find it, so it must be new 
        $subject = new Subject;
        $subject->setName($str);
        $this->em->persist($subject);

        return $subject;
    }
}