Symfony entity form with dynamic fields from another entity

Question

Question:

Consider following Order form:

META DATA:

Title:       [________]

REQUIREMENTS:

What size?  [_] Small    [_] Medium    [_] Large
What shape?  [_] Circle    [_] Square    [_] Triangle
.
.
.

How can I generate the form?

Constraints:

• size & shape & ... should be retrieved form another entity named: Requirement.

What I think:

Order

Class Order
{
    private $title;

    //OneToMany(targetEntity="Requirement", mappedBy="order")
    private $requirements;

    public function __construct()
    {
        $this->requirements = new ArrayCollection();
    }
}

Requirement

Class Requirement
{
    private $title;

    //ManyToOne(targetEntity="Order", inversedBy="requirements")
    private $order;

    //OneToMany(targetEntity="Selection", mappedBy="selections")
    private $selections;

    public function __construct()
    {
        $this->selections = new ArrayCollection();
    }

}

Selection

Class Selection
{
    private $title;

    //ManyToOne(targetEntity="Requirement", inversedBy="selections")
    private $requirement;
}

OrderType

class OrderType extends AbstractType
{
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'allow_extra_fields' => true
        ));
        $resolver->setRequired('em');
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $em = $options['em'];
        $requirements = $em->getRepository(Requirement::class)->findAll();

        $form = $builder
            ->add('title', TextType::class, array());

        foreach ($requirements as $requirement) {

            $args['label']         = $requirement->getTitle();
            $args['expanded']      = true;
            $args['multiple']      = true;
            $args['mapped']        = false;
            $args['class']         = Selection::class;
            $args['choice_label']  = 'title';
            $args['choice_value']  = 'id';
            $args['query_builder'] = call_user_func(function (EntityRepository $er, $requirement) {
                    return $er->createQueryBuilder('s')
                        ->where('s.requirement = :requirement')
                        ->setParameter('requirement', $requirement)
                },$em->getRepository($args['class']), $requirement);

            $form
                ->add($requirement->getTitle(), EntityType::class, $args);

        }

        $form
            ->add('submit', SubmitType::class, array(();

        return $form;
    }
}

Problem:

This works nice and I can persist new Orders. The problem is with editing the Order, as

$form->createView()

while filling order title correctly, would not update the selections (checkboxes). I don't even know if this is the right way of doing this on Symfony. I don't know how can I refill selected checkboxes while rendering the edit entity form.

Notice:

I don't think if Symfony Form Collections is a good idea, as the requirements are changing in time and could not be hardcoded as FormTypes, thus should be stored in database.