Allow ResultSetMapping to hydrate unmapped entity parameter

[Jarod-MIDY]

Feature Request

Q	A
New Feature	yes
RFC	yes
BC Break	not sure

Summary

While using the ResultSetMapping object to hydrate a doctrine-managed entity from a native SQL request I could not hydrate parameters that do not correspond to a db column.

the SQL query counts the total number of pages by book. I don't want to add a nb_pages column in my book table.

the entity :

#[ORM\Entity(repositoryClass: BookRepository::class)]
class Book {

    #[ORM\Column(length: 255)]
    public string $title = ''; // this I can hydrate as expected

    public ?int $nbPages = null; // this doesn't seem to be hydratable with ResultSetMapping
   [...]
}

the mapping :

        $rsm = new ResultSetMapping();
        $rsm->addEntityResult(Book::class, 'project')
            ->addMetaResult('project', 'project_id', 'id', true)
            ->addFieldResult('project', 'project_id', 'id')
            ->addFieldResult('project', 'project_created_at', 'createdAt')
            ->addFieldResult('project', 'project_updated_at', 'updatedAt')
            ->addFieldResult('project', 'project_title', 'title');
        $rsm->addJoinedEntityResult(Chapter::class, 'c', 'project', 'chapters')
            ->addMetaResult('c', 'chapter_id', 'id', true)
            ->addFieldResult('c', 'chapter_id', 'id')
            ->addFieldResult('c', 'chapter_title', 'title')
            ->addFieldResult('c', 'chapter_position', 'position')
            ->addFieldResult('c', 'chapter_created_at', 'createdAt')
            ->addFieldResult('c', 'chapter_updated_at', 'updatedAt');
        $rsm->addJoinedEntityResult(Page::class, 'p', 'c', 'pages')
            ->addFieldResult('p', 'page_id', 'id')
            ->addFieldResult('p', 'page_created_at', 'createdAt')
            ->addFieldResult('p', 'page_updated_at', 'updatedAt')
            ->addFieldResult('p', 'page_position', 'position')
            ->addFieldResult('p', 'page_published_at', 'publishedAt')
            ->addFieldResult('p', 'page_title', 'title');

// I tried to hydrate it as if It were a DTO as explained in the documentation but this did not work :
        $rms->addScalarResult('total_pages', 'nbPages');
        $rsm->newObjectMappings['nbPages']  = [
           'className' => Book::class,
           'objIndex'  => 0,
           'argIndex' => 'nbPages'
        ];

[soullivaneuh]

The logic described above produce the following error:

ErrorException:
Warning: Undefined array key "nbPages"

  at vendor/doctrine/orm/src/Internal/Hydration/AbstractHydrator.php:549
  at Doctrine\ORM\Internal\Hydration\AbstractHydrator->hydrateColumnInfo('total_pages')
     (vendor/doctrine/orm/src/Internal/Hydration/AbstractHydrator.php:414)

It is because the nbPages is not part of the class metadata used by AbstractHydrator.

The ColumnResult annotation looks to be the solution in order to make it considering the field to the metadata as a result only property.

However, this annotation was removed before the attribute migration by #7095 and #10114 without any explanation of why referenced.