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;
    }
}

services.yml

hidden_entity_form_type:
    class: \Acme\DemoBundle\Form\Type\HiddenEntityType
    arguments:
      - @doctrine.orm.entity_manager
    tags:
      - { name: form.type, alias: entity_hidden }

SomeController.php

<?php

// ... 

$form = $this->createFormBuilder(array('user' => $user))
->add('user', 'hidden_entity', array(
  'class' => '\Acme\DemoBundle\Entity\User'
))
->getForm()
;