EntityHiddenType.php

<?php

namespace Dpn\ToolsBundle\Form\Type;

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

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

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

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

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'class' => null,
            'invalid_message' => 'The entity does not exist.',
        ));
    }

    public function getParent()
    {
        return 'hidden';
    }

    public function getName()
    {
        return 'entity_hidden';
    }
}

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;

    public function __construct(ObjectManager $objectManager, $class)
    {
        $this->objectManager = $objectManager;
        $this->class = $class;
    }

    public function transform($entity)
    {
        if (null === $entity) {
            return;
        }

        return $entity->getId();
    }

    public function reverseTransform($id)
    {
        if (!$id) {
            return null;
        }

        $entity = $this->objectManager
                       ->getRepository($this->class)
                       ->find($id);

        if (null === $entity) {
            throw new TransformationFailedException();
        }

        return $entity;
    }
}