Custom choices on form with data_class - php

Question

I have created a FormType that get's a data_class set in the defaults in the configureOptions class. It looks a bit like this
<?php
namespace tzfrs\AppBundle\Form\Type;
use tzfrs\AppBundle\Model\Car;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
/**
* Class CarType
* #package tzfrs\AppBundle\Form\Type
*/
class CarType extends AbstractType
{
/**
* {#inheritdoc}
*/
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder->add('specification', ChoiceType::class, [
'choices' => [1 => 1, 2 => 2, 3 => 3]
]);
}
/**
* #inheritdoc
*/
public function configureOptions(OptionsResolver $resolver)
{
$resolver->setDefaults([
'data_class' => Car::class
]);
}
}
When I now create the form like this
$carForm = $this->createForm(CarType::class, $car);
and want to use it I get the following error:
The value of type "object" cannot be converted to a valid array key.
I think the reason for this, is because my Car class has a property specification with getters/setters and therefore returns a Specification object. However, my Specification object has a __toString() method.
Car.php
/**
* #var Specification
*/
protected $specification;
public function getSpecification(): Specification
{
return $this->specification;
}
public function setSpecification(Specification $specification): Car
{
$this->specification = $specfification;
return $this;
}
Specification.php
/**
* Returns a string implementation of the current class
*
* #return string
*/
public function __toString(): string
{
return sprintf(
'HP: %s,CCM: %s',
$this->getHorsePower(),
$this->getCapacity()
);
}
How would I achieve overriding my choices for a form field which uses a custom Model? I can't use the EntityType because it's not a Doctrine Model.
When I build my form like this
$carForm = $this->createForm(Car::class);
then it works, but I need the Car object inside the Form, because of another field I'm adding in the buildForm method