Hidden EntityType field

config.yml

services:

    form.type.entity_hidden:
        class: PlatformBundle\Form\Type\EntityHiddenType
        arguments:
            - '@doctrine.orm.entity_manager'
        tags:
            - { name: form.type }

EntityHiddenType.php

<?php

namespace PlatformBundle\Form\Type;

use PlatformBundle\Form\DataTransformer\EntityToIdTransformer;
use Doctrine\Common\Persistence\ObjectManager;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\HiddenType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class EntityHiddenType extends AbstractType
{
    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 configureOptions(OptionsResolver $resolver)
    {
        $resolver
            ->setRequired(['class'])
            ->setDefaults(['invalid_message' => 'The entity does not exist.']);
    }

    public function getParent()
    {
        return HiddenType::class;
    }
}

EntityToIdTransformer.php

<?php

namespace PlatformBundle\Form\DataTransformer;

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

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 null;
        }

        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;
    }
}