How to make association between two DTO classes using stateOptions

[mleko64]

Supose I have two Entity classes:

#[ORM\Entity]
class BookEntity
{
    public function __construct(
        #[ORM\Id]
        #[ORM\Column]
        public int $id,
        #[ORM\Column]
        public string $title,
        #[ORM\ManyToOne(targetEntity: AutorEntity::class, inversedBy: 'books')]
        public Author $author,
    ) {}
}

#[ORM\Entity]
class AuthorEntity
{
    public function __construct(
        #[ORM\Id]
        #[ORM\Column]
        public int $id,
        #[ORM\Column]
        public string $name,
        #[ORM\OneToMany(targetEntity: BookEntity::class, mappedBy: 'author')]
        public Collection $books;
    ) {
        $this->books = new ArrayCollection();
    }
}

And also I have two DTO classes:

#[ApiResource(
    operations: [
        new Get(),
        new GetCollection(),
    ],
    stateOptions: new Options(entityClass: BookEntity::class)
)]
class BookApi
{
    public int $id;
    public string $title;
    public AuthorApi $author;
}

#[ApiResource(
    operations: [
        new Get(),
        new GetCollection(),
    ],
    stateOptions: new Options(entityClass: AuthorEntity::class)
)]
class AuthorApi
{
    public int $id;
    public string $name;

    /**
     * array<BookApi>
     */
    public array $books;
}

1. How to handle this situation to map $author as AuthorApi object (in BookApi) and $books as collection (array) of BookApi objects (in AuthorApi)?
2. How to tell API Platform to expose $books field as collection of BookApi, but should be fetch BookEntity objects behind the scene?

I don't want mix up entities with DTO classes, for example:

class BookApi
{
    public int $id;
    public string $title;
    public AuthorEntity $author; // I don't want mix-up Entity with DTO
}

I tried to use "symfony/object-mapper" but no chance so far.
Thanks for Your help.

[solariox]

I am struggling with the exact same problem for weeks now.

I've summarized some of my research here, if you want to take a look:
https://gist.github.com/solariox/2e6726afe36ac62ad9bf005a69fb2cf0

For inputs, we don't have a satisfying result right now.
For simple case, ObjectMapper is working fine. But as soon as we want to handle sub-resource, like giving IRI in a POST, we end up to use a custom processor, because we cannot succeed other way.

For Outputs, we've end up to use a mix of serializer groups, with custom source/target attributes if needed.