// src/Entity/BlogPostTranslation.php
namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;
use App\ValueObject\Locale;

#[ORM\Entity]
#[ORM\UniqueConstraint(fields: ['locale', 'slug'])]
class BlogPostTranslation
{
    #[ORM\Id]
    #[ORM\ManyToOne(targetEntity: BlogPost::class, inversedBy: 'translations')]
    #[ORM\JoinColumn(nullable: false, onDelete: 'CASCADE')]
    private BlogPost $post;

    #[ORM\Id]
    #[ORM\Column(type: 'locale', nullable: false)]
    private Locale $locale;

    #[ORM\Column(nullable: false)]
    private string $slug;

    // ...
}

// src/Form/Data/BlogPostTranslationFormData.php
namespace App\Form\Data;

use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
use Symfony\Component\Uid\Uuid;
use Symfony\Component\Validator\Constraints\NotBlank;
use App\Entity\BlogPostTranslation;

#[UniqueEntity(
    fields: ['locale', 'slug'],
    message: 'This slug is already used by another post in this language.',
    entityClass: BlogPostTranslation::class,
    // `identifierFieldNames` cannot be used, as, even though the `BlogPostTranslation` entity
    // would be found, the equality check of `postId === (string) post` would fail.
    // identifierFieldNames: [
    //     'postId' => 'post',
    //     'locale' => 'locale',
    // ],
    // instead, a comparator can be provided.
    comparator: [BlogPostTranslationFormData::class, 'compare'],
    errorPath: 'slug'
)]
class BlogPostTranslationFormData
{
    public function __construct(
        public readonly Uuid $postId,
        public readonly string $locale,

        #[NotBlank]
        public ?string $slug = null,
    ) {}

    public static function compare(self $subject, BlogPostTranslation $foundEntity): bool
    {
        return $foundEntity->getPost()->getId()->equals($this->postId)
            && $foundEntity->getLocale()->toValue() === $this->locale;
    }
}