HiddenEntityType.php

<?php
namespace Absolvent\AbsolventBundle\Form\Type;

use Absolvent\AbsolventBundle\Form\DataTransformer\EntityToIntTransformer;
use Doctrine\Common\Persistence\ObjectManager;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormRegistryInterface;
use Symfony\Component\OptionsResolver\Options;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class HiddenEntityType extends AbstractType
{
    /**
     * @var ObjectManager
     */
    protected $om;

    /**
     * @var FormRegistryInterface
     */
    protected $registry;

    public function __construct(ObjectManager $om, FormRegistryInterface $registry)
    {
        $this->om = $om;
        $this->registry = $registry;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $parentClassProperty = $builder->getName();

        if (false == $options['class']) {
            $parentClass = $builder->getParent()->getDataClass();
            $guessedType = $this->registry->getTypeGuesser()->guessType($parentClass, $parentClassProperty);
            $guessedOptions = $guessedType->getOptions();

            if (false === array_key_exists('class', $guessedOptions)) {
                throw new FormException(sprintf(
                    'Cannot guess %s entity class',
                    $parentClassProperty
                ));
            }
            $options['class'] = $guessedOptions['class'];
        }

        $transformer = new EntityToIntTransformer($this->om);
        $transformer->setEntityClass($options['class']);
        $transformer->setEntityType($parentClassProperty);

        $builder->addModelTransformer($transformer);
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults([
            'class' => null
        ]);
    }

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

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