ObjectMapper wrong association with child entity

[Develog]

API Platform version(s) affected: 4.2.14

Description

Hi !
When using ObjectMapper, child DTO management does not seem to work on a GET / GET Collection method.
When I have a Parent A entity that contains a child Aa and I have a DTO corresponding to these two entities, DtoA and DtoAa, then in my DtoA I am unable to map the child Dto.
Or perhaps I am missing something in my configuration or in my understanding of how to use ObjectMapper with ApiPlatform.
Maybe it's more related to ObjectMapper than ApiPlatform ?

How to reproduce

With this example, I get a mapping error on the category relationship. It seems that ObjectMapper is unable to connect to my resource.
Expected argument of type "?App\Api\ApiResource\Category", "App\Entity\Category" given at property path "category".

Entity Product :

declare(strict_types=1);

namespace App\Entity;

use Doctrine\DBAL\Types\Types;
use Doctrine\ORM\Mapping\Column;
use Doctrine\ORM\Mapping\Entity;
use Doctrine\ORM\Mapping\GeneratedValue;
use Doctrine\ORM\Mapping\Id;
use Doctrine\ORM\Mapping\JoinColumn;
use Doctrine\ORM\Mapping\ManyToOne;
use Gedmo\Mapping\Annotation\Translatable;
use Symfony\Component\Validator\Constraints\NotBlank;
#[Entity]
class Product
{
    #[Id]
    #[GeneratedValue]
    #[Column(type: Types::INTEGER)]
    private ?int $id = null;

    #[ManyToOne(targetEntity: Category::class)]
    #[JoinColumn(nullable: true, onDelete: 'SET NULL')]
    private ?Category $category = null;

    #[Column(type: Types::STRING)]
    private string $name;

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

    public function getCategory(): ?Category
    {
        return $this->category;
    }

    public function setCategory(?Category $category): void
    {
        $this->category = $category;
    }

    public function getName(): string
    {
        return $this->name;
    }

    public function setName(string $name): void
    {
        $this->name = $name;
    }
}

Entity Category :

<?php

declare(strict_types=1);

namespace App\Entity;

use Doctrine\DBAL\Types\Types;
use Doctrine\ORM\Mapping\Column;
use Doctrine\ORM\Mapping\Entity;
use Doctrine\ORM\Mapping\GeneratedValue;
use Doctrine\ORM\Mapping\Id;

#[Entity]
class Category
{
    #[Id]
    #[GeneratedValue]
    #[Column(type: Types::INTEGER)]
    private ?int $id = null;

    #[Column(name: 'label', type: Types::STRING, nullable: false)]
    private ?string $label = null;

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

    public function getLabel(): ?string
    {
        return $this->label;
    }

    public function setLabel(?string $label): void
    {
        $this->label = $label;
    }
}

DTO Product :

declare(strict_types=1);

namespace App\Api\ApiResource;

use ApiPlatform\Doctrine\Orm\State\Options;
use ApiPlatform\Metadata\ApiResource;
use ApiPlatform\Metadata\Get;
use App\Entity\Product as ProductEntity;
use Symfony\Component\ObjectMapper\Attribute\Map;

#[ApiResource(
    operations: [
        new Get(
            uriTemplate: '/products/{id}',
        ),
    ],
    stateOptions: new Options(entityClass: ProductEntity::class)
)]
#[Map(source: ProductEntity::class)]
class Product
{
    public int $id;

    public string $name;

    public ?Category $category = null;
}

DTO Category

<?php

declare(strict_types=1);

namespace App\Api\ApiResource;

use App\Entity\Category as CategoryEntity;
use Symfony\Component\ObjectMapper\Attribute\Map;

#[Map(source: CategoryEntity::class)]
class Category
{
    public int $id;
    public string $label;
}

Thank you for your help.

[soyuka]

I've reproduced this issue with a failing test on branch fix/issue-7801-nested-dto-mapping.

The root cause: when using #[Map(source: EntityClass::class)] on DTOs (without #[Map(target: DtoClass::class)] on entities), the Symfony ObjectMapper can't determine the target class for nested objects. The metadata factory returns empty for entities without #[Map] attributes, so nested entity→DTO mapping is skipped and the raw entity is passed through, causing the type error.

The fix depends on symfony/symfony#63705 which enhances ReverseClassObjectMapperMetadataFactory with runtime nested mapping discovery. Once that PR is merged and API Platform wires ReverseClassObjectMapperMetadataFactory, this will work.

Workaround: add #[Map(target: DtoClass::class)] on your entities (bidirectional mapping), which works with symfony/object-mapper >= 8.1.

Note: This comment what generated by Claude.

I still want to look deeper into this, I think there must be a way to improve how ReverseClassObjectMapperMetadataFactory is working on Symfony and to help with setting Map only on sources.