ModelTransformer

EntityIdentifierType.php

<?php

namespace Core\MediaBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Core\MediaBundle\Form\DataTransformer\EntityToIdentifierTransformer;
use Doctrine\Common\Persistence\ObjectManager;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class EntityIdentifierType extends AbstractType
{
    /**
     * @var ObjectManager
     */
    private $om;

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

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

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            // messages
            'invalid_message' => 'The selected entity does not exist',
        ));

        $resolver->setRequired(array(
            'class'
        ));
    }

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

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

EntityToIdentifierTransformer.php

<?php

namespace Core\MediaBundle\Form\DataTransformer;

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

class EntityToIdentifierTransformer implements DataTransformerInterface
{
    /**
     * @var ObjectManager
     */
    private $om;

    /**
     * @var string
     */
    private $classname;

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

    /**
     * Transforms an object (entity) to a string (id).
     *
     * @param  Entity|null $entity
     * @return string
     */
    public function transform($entity)
    {
        if (null === $entity) {
            return "";
        }

        return $entity->getId();
    }

    /**
     * Transforms a string (id) to an object (entity).
     *
     * @param  string $id
     * @return Entity|null
     * @throws TransformationFailedException if object (entity) is not found.
     */
    public function reverseTransform($id)
    {
        if (!$id) {
            return null;
        }

        $entity = $this->om
            ->getRepository($this->classname)
            ->find($id)
        ;

        if (null === $entity) {
            throw new TransformationFailedException(sprintf(
                'The entity with "%s" id could not be found',
                $id
            ));
        }

        return $entity;
    }
}

services.yml

parameters:
#    core_media.example.class: Core\MediaBundle\Example

services:
#    core_media.example:
#        class: %core_media.example.class%
#        arguments: [@service_id, "plain_value", %parameter%]
    core_media.type.entity_id:
        class: Core\MediaBundle\Form\Type\EntityIdentifierType
        arguments: ["@doctrine.orm.entity_manager"]
        tags:
            - { name: form.type, alias: entity_id }