Querying a child relation of the same entity type not possible in GraphQL

[NicoHaase]

API Platform version(s) affected: 3.0.8

Description
In my application, I use a parent/child relation with entities of the same type, like the following:

<?php

declare(strict_types=1);

namespace App\Entity\QualityManual;

use ApiPlatform\Metadata\GraphQl\QueryCollection;
use ApiPlatform\Metadata\GraphQl\Query;
use ApiPlatform\Metadata\ApiResource;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Bridge\Doctrine\IdGenerator\UuidGenerator;
use Symfony\Component\Serializer\Annotation\Groups;

#[ApiResource(
    graphQlOperations: [
        new Query(name: 'item_query'),
        new QueryCollection(name: 'collection_query'),
    ]
)]
#[ORM\Entity]
class Chapter
{
    #[ORM\Id]
    #[ORM\Column(type: 'guid', unique: true)]
    #[ORM\GeneratedValue(strategy: 'CUSTOM')]
    #[ORM\CustomIdGenerator(class: UuidGenerator::class)]
    private ?string $id = null;

    #[ORM\OneToMany(mappedBy: 'parent', targetEntity: Chapter::class, cascade: ['remove'])]
    #[ORM\OrderBy(['position' => 'ASC'])]
    private Collection $children;

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

    public function getId(): ?string
    {
        return $this->id;
    }
}

When I try to select a single Chapter entity and the number of it's children, the result is not built properly. As far as I can debug this, it's a problem of these lines:

core/src/GraphQl/Type/FieldsBuilder.php

Lines 265 to 268 in e2753fa

	if ($resourceClass && $rootOperation->getClass() && $this->resourceClassResolver->isResourceClass($resourceClass) && $rootOperation->getClass() !== $resourceClass) {
	$resourceMetadataCollection = $this->resourceMetadataCollectionFactory->create($resourceClass);
	$resourceOperation = $resourceMetadataCollection->getOperation($isCollectionType ? 'collection_query' : 'item_query');
	}

Due to the last condition, the $resourceOperation is not properly set to a collection operation, but to an item operaion. This results in an error in

core/src/GraphQl/Resolver/Factory/CollectionResolverFactory.php

Lines 59 to 62 in e2753fa

	$collection = ($this->readStage)($resourceClass, $rootClass, $operation, $resolverContext);
	if (!is_iterable($collection)) {
	throw new \LogicException('Collection from read stage should be iterable.');
	}

as the resulting collection is not a collection, but null (even if there are entities matching this criteria).

How to reproduce

• use the entity definition as above
• run a query like { chapter(id: "/api/chapters/chapterId") { id, children { totalCount } } }

Additional Context

• this worked properly in ApiPlatform v2.7 (with metadata_backward_compatibility_layer=true) and below
• this looks similar to GraphQL Nested Children error ( is used outside the scope of its declaration ) #5007

[alanpoulain]

It would help if you could create a PR with a failing Behat test on the 3.0 branch.

[develth]

So, you do not get an error like in #5007 ?