Sylius - Overriding CartItemType breaks the Product show view - sylius

Question

Sylius 1.0.0-dev
I'm trying to modify the Quantity field of CartItemType.
Following the old docs I have created a new FormType on my Bundle and extends from original in Symfony/Bundle/CartBundle/Form/Type/CartItemType.
My custom CartItemType shows like this:
use Sylius\Bundle\CartBundle\Form\Type\CartItemType as BaseType;
use Symfony\Component\Form\FormBuilderInterface;
class CartItemType extends BaseType
{
/**
* {#inheritdoc}
*/
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->remove('quantity')
->add('quantity', 'hidden', [
'attr' => ['min' => 1, 'max' => 1],
'data' => '1',
'label' => 'sylius.form.cart_item.quantity',
])
->setDataMapper($this->orderItemQuantityDataMapper);
}
}
I want you can't buy more than one product quantity per order.
My config seems liike this:
sylius_cart:
resources:
cart_item:
classes:
form:
default: Dinamic\Bundle\SyliusRibBundle\Form\Type\CartItemType
When I open some product view i'm getting this error:
The option "product" does not exist. [...] in #SyliusShop/Product/Show/_inventory.html.twig at line 4
Any idea about why this is happening?

Answer 1

Instead of extending CartBundle CartItemType extend using this:
use Sylius\Bundle\CoreBundle\Form\Type\CartItemType as BaseType;
The CoreBundle CartItemType class extends the CartBundle CartItemType, so if you extend the wrong class it will break everything

Answer 2

Don't extend Symfony form types that way - extend Symfony\Component\Form\AbstractType instead, and implement getParent() in your custom type:
public function getParent()
{
return BaseType::class;
}
That way, the parent's options will be available to your custom type as well, and everything will be properly rendered/initialized.
Edit:
Technically, both approaches should work. However, using inheritance will completely override the parent form's behavior, while using getParent() will add your custom behavior to the parent's form.
getParent() will tell Symfony to build the form defined in that method first, and then use that as the base for your own implementation. So when using that, the FormBuilder in your buildForm() method will already contain the complete parent form, ready for your own modifications, the OptionsResolver in your configureOptions() method will already have the parent form's options and defaults defined, etc. which appears to be exactly what you want (otherwise your call to $builder->remove() makes very little sense). When using inheritance, you'll have to take care to do everything the parent form does which you want to do as well, which might very well break when the parent form changes.
Another difference is in Symfony's form extensions. When implementing getParent(), any form extensions applied to the parent form will be applied to your custom form as well. When using inheritance, that won't be the case.
The first point is easy to work with (parent::buildForm() will ensure you have the correct fields, for instance), but the second is not as easy. Generally you want these extensions to apply to your custom form type as well - in this case there's actually a decent chance the error is caused by a form extension not being applied to your custom type.
So basically, use getParent() whenever possible. Use inheritance for form types only if you want to completely override behaviour or you explicitly want form extensions not to apply to your custom type.