Symfony2: Specify form type as a field type of another form

Building on my comment above, I'd suggest not doing the data transformation in the form type (though it's certainly possible), but rather in the encapsulating model using the json_array type.

This way only that model actually knows how the data will be persisted.

The two relevant models:

src/AppBundle/Entity/FooModel.php

<?php
declare(strict_types=1);

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity()
 */
class FooModel
{
    // other properties (firstName, lastName, ...)

    /**
     * @var array
     *
     * @ORM\Column(type="json_array")
     */
    private $checklist = [];

    /**
     * @param ChecklistModel $checklist
     */
    public function setChecklist(ChecklistModel $checklist)
    {
        $this->checklist = $checklist->toArray();
    }

    /**
     * @return ChecklistModel
     */
    public function getChecklist(): ChecklistModel
    {
        return ChecklistModel::fromArray($this->checklist);
    }
}

with ChecklistModel implementing the above mentioned methods:

src/AppBundle/Entity/ChecklistModel.php

<?php
declare(strict_types=1);

namespace AppBundle\Entity;

class ChecklistModel
{
    // properties and getters/setters

    /**
     * @param array $data
     *
     * @return ChecklistModel
     */
    public static function fromArray(array $data): ChecklistModel
    {
        $result = new self;

        foreach (get_class_vars(self::class) as $k => $v) {
            if (isset($data[$k])) {
                $result->$k = $data[$k];
            }
        }

        return $result;
    }

    /**
     * @return array
     */
    public function toArray()
    {
        return get_object_vars($this);
    }
}

The form types:

src/AppBundle/Form/FooFormType.php

<?php
declare(strict_types=1);

namespace AppBundle\Form;

use AppBundle\Entity\FooModel;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\Extension\Core\Type as FormType;

class FooFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('firstName', FormType\TextType::class)
            ->add('lastName', FormType\TextType::class)
            ->add('checklist', ChecklistFormType::class)
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => FooModel::class,
            'empty_data' => new FooModel(),
        ]);
    }
}

src/AppBundle/Form/ChecklistFormType.php

<?php
declare(strict_types=1);

namespace AppBundle\Form;

use AppBundle\Entity\ChecklistModel;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\Extension\Core\Type as FormType;

class ChecklistFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('clientSatisfied', FormType\ChoiceType::class, [
                'choices' => [
                    'yes' => 'yes',
                    'no' => 'no'
                ],
                'choices_as_values' => true,
            ])
            ->add('clientNewCustomer', FormType\ChoiceType::class, [
                'choices' => [
                    'yes' => 'yes',
                    'no' => 'no'
                ],
                'choices_as_values' => true,
            ])
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => ChecklistModel::class,
            'empty_data' => new ChecklistModel(),
        ]);
    }
}

example usage

public function indexAction(Http\Request $request)
{
    $em = $this->getDoctrine()->getManager();

    $data = new Entity\FooModel();

    $form = $this
        ->createForm(FooFormType::class, $data)
        ->handleRequest($request)
    ;

    if ($form->isSubmitted() && $form->isValid()) {
        $em->persist($data);
        $em->flush();
    }

    return $this->render('default/index.html.twig', [
        'form' => $form->createView(),
        'data' => $form->getData(),
    ]);
}

This way the ChecklistFormType dose not need to know anything about the data being json or something else. A ChecklistModel goes in and comes out, no surprises.

That said, embeddables might be a better choice here.