Symfony 3.4表单中的CollectionType数组

There is an example of how to do what you're looking to achieve in the Symfony Documentation on How to Embed a Collection of Forms.

For your specific use case, you will want to create a UserCartsForm and a Separate CartsForm.

In your UserCart add the carts field as a CollectionType. Symfony will then process the field as a series of forms.

src/AppBundle/Form/UserCart.php

namespace AppBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type as FormType;

class UserCartsForm extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
         $builder->add('carts', FormType\CollectionType::class, [
             'label' => false,
             'entry_type' => CartsForm::class,
             'entry_options' => array('label' => false),
         ]);
    }

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

Add the fields you want editable on your form to the CartsForm

src/AppBundle/Form/CartsForm.php

namespace AppBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type as FormType;

class CartsForm extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
         $builder->add('quantity', FormType\IntegerType::class, [
           'label' => false
            //...
         ]);
         //...
    }

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

In your controller, reference the user entity as your UserCartsForm data.

src/AppBundle/Controller/DefaultController.php

namespace AppBundle\Controller;

use AppBundle\Form\UserCartsForm;

class DefaultController extends Controller
{

   /**
    * @Route('/{id}/user-carts')
    */
   public function userCartsAction(Request $request, User $user)
   {
      $form = $this->createForm(UserCartsForm::class, $user);
      $form->handleRequest($request);
      if($form->isSubmitted() && $form->isValid())
      {
         //... process entity
         //$this->getDoctrine()->getManager()->flush($carts);
         return $this->redirectToRoute('some_route');
      }

      return $this->render('user_carts_form.html.twig', [
          'form' => $form
      ]);
   }
}

Then you should be able to easily retrieve the data from your twig template to render as you would like.

app/Resources/views/user_carts_form.html.twig

{% form_start(form) %}
   <table>
   <thead>
   <tr>
       <td>Name</td>
       <td>Quantity</td>
       <td></td>
   </tr>
   </thead>
   <tbody>
   {% for cart in form.carts %}
   {% set cartEntity = cart.vars.data %}
   <tr>
       <td>{{ cartEntity.product.name }}</td>
       <td>{{ form_widget(cart.quantity) }}</td>
       <td><a class="button" href="{{ path('remove_cart_action', { id: cartEntity.id }) }}">Delete <icon/></a></td>
   <tr>
   {% endfor %}
   </tbody>
   </table>
   <button type="submit">Submit</button>
{% form_end(form) %}

---

Update for Entity Constraints

By default Symfony will use all constraints (Default) assigned to the entity when validating your form causing $form->isValid() to return false.

https://symfony.com/doc/3.4/validation/groups.html

If no groups are specified, all constraints that belong to the group Default will be applied.

To resolve the issue, use Validation Groups to separate the entity constraints and declare the desired groups on their respective form(s).

Example:

src/AppBundle/Entity/User.php

namespace AppBundle\Entity;

use Symfony\Component\Validator\Constraints as Assert;

/**
 * @ORM\Entity
 */
class User
{

    /**
     * @Assert\NotBlank(groups={"registration"})
     */
    private $username;

    //...
}

Then to use your validation group(s) on the desired form, in this case RegistrationForm, you declare the desired group in the AbstractTye::configureOptions as one of the OptionsResolver:$defaults.

src/AppBundle/Form/RegistrationForm.php

namespace AppBundle\Form;

use Symfony\Component\Form\AbstractType;

class RegistrationForm extends AbstractType
{
    //...

     public function configureOptions(OptionsResolver $resolver)
     {
         $resolver->setDefaults([
             'data_class' => User::class,
             'validation_groups' => ['registration']
         ]);
     }

}

Now the User::NotBlank constraint will only apply for the RegistrationForm::isValid() or any other form that declare the registration validation group.