[php] Hidden field for Symfony2 entities

EntityHiddenType.php

<?php

namespace Dpn\ToolsBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Doctrine\Common\Persistence\ObjectManager;
use Dpn\ToolsBundle\Form\DataTransformer\EntityToIdTransformer;

class EntityHiddenType extends AbstractType
{
    /**
     * @var ObjectManager
     */
    protected $objectManager;

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

    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $transformer = new EntityToIdTransformer($this->objectManager, $options['class']);
        $builder->addModelTransformer($transformer);
    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver
            ->setRequired(array('class'))
            ->setDefaults(array(
                'invalid_message' => 'The entity does not exist.',
            ))
        ;
    }

    /**
     * {@inheritdoc}
     */
    public function getParent()
    {
        return 'Symfony\Component\Form\Extension\Core\Type\HiddenType';
    }
}

EntityToIdTransformer.php

<?php

namespace Dpn\ToolsBundle\Form\DataTransformer;

use Symfony\Component\Form\DataTransformerInterface;
use Symfony\Component\Form\Exception\TransformationFailedException;
use Doctrine\Common\Persistence\ObjectManager;

class EntityToIdTransformer implements DataTransformerInterface
{
    /**
     * @var ObjectManager
     */
    protected $objectManager;

    /**
     * @var string
     */
    protected $class;

    /**
     * Constructor
     * @param ObjectManager $objectManager
     * @param string        $class
     */
    public function __construct(ObjectManager $objectManager, $class)
    {
        $this->objectManager = $objectManager;
        $this->class = $class;
    }

    /**
     * {@inheritdoc}
     */
    public function transform($entity)
    {
        if (null === $entity) {
            return;
        }
        return $entity->getId();
    }

    /**
     * {@inheritdoc}
     */
    public function reverseTransform($id)
    {
        if (!$id) {
            return null;
        }
        $entity = $this->objectManager
                       ->getRepository($this->class)
                       ->find($id);
        if (null === $entity) {
            throw new TransformationFailedException();
        }
        return $entity;
    }
}

ExampleType.php

<?php

namespace Dpn\ToolsBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Dpn\ToolsBundle\Form\Type\EntityHiddenType;

class ExampleType extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('category', EntityHiddenType::class, array(
                'label' => false,
                'class' => 'Dpn\ToolsBundle\Entity\Category'
            ))
        ;
    }

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

services.yml

services:
    dpn.type.entity_hidden:
       class: Dpn\ToolsBundle\Form\Type\EntityHiddenType
       arguments: ["@doctrine.orm.entity_manager"]
       tags:
            - { name: form.type }