以编程方式将表单附加到Symfony中的模型

A Syfony Forms component is usually bound to some model either by defining default_mapping option or passing the model object to $formBuilder-createForm($formType, $modelObject).

I am building a form that would initially only have one field: e-mail address. If the input address is found in the database, a transactional message with a login link should be sent. If such user does not exist, more fields must be added to the form so we can register said user.

My approach now uses a unbound form with events to check for the existense for the user.

How can I programatically assign the form the my User model if the user is not found to enable validation? Is there a better approach than using events?

<?php

namespace App\Form;

use App\Entity\User;
use Doctrine\ORM\EntityManagerInterface;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\CheckboxType;
use Symfony\Component\Form\Extension\Core\Type\EmailType;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\OptionsResolver\OptionsResolver;

class RegistrationType extends AbstractType
{
    private $em;

    public function __construct(EntityManagerInterface $em) {
        $this->em = $em;
    }


    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('email', EmailType::class, ['label' => false, 'required' => true, 'mapped' => false]);
        $builder->addEventListener(FormEvents::PRE_SUBMIT, array($this, 'onPreSubmitData'));
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => null,
        ]);
    }

    public function onPreSubmitData(FormEvent $event) {
        $form = $event->getForm();
        $data = $event->getData();
        $users = $this->em->getRepository(User::class);
        dump($event->getForm()->isSubmitted());

        if ($data['email'] && !$user = $users->findOneByEmail($data['email'])) {
            $form->add('name', TextType::class);
            $form->add('surname', TextType::class);
            $form->add('tos', CheckboxType::class, ['required' => true, 'mapped' => false]);
            $form->add('consentOn', CheckboxType::class);
            $form->add('submit', SubmitType::class);
        }

        if (isset($data['submit'])) {
            // Create new user
        }
    }
}