We're already using this on a project and it works quite well.

How do you use it? How is this supposed when using sources only?
Ive tried using MapCollection transformer but it will throw exception:

class UserDto {
/**
     * @var OrganizationMembershipDto[]
     */
    #[Map(transform: new MapCollection(), source: "memberships")]
    public array $memberships;
    
}

$dto = $this->mapper->map($user, UserDto::class);

will throw:

Symfony\Component\ObjectMapper\Exception\MappingException : Mapping target not found for source "App\Entity\User\OrganizationMembership".
 /var/www/vendor/symfony/object-mapper/ObjectMapper.php:58
 /var/www/src/Dto/Response/ObjectMapper/Transformer/MapCollection.php:30
 /var/www/vendor/symfony/object-mapper/ObjectMapper.php:225
 /var/www/vendor/symfony/object-mapper/ObjectMapper.php:259
 /var/www/vendor/symfony/object-mapper/ObjectMapper.php:176
 /var/www/vendor/symfony/object-mapper/ObjectMapper.php:139

Like ive mentioned in #61116 theres is currently no API in object mapper to provide metadata to transformers and without this transformers are unable to guess type of target value.
Having to set the target on the source breaks SRP and erodes the actual purpose of object mapper.

TDLR; Only works when mapping target is defined on source.

@crtl related to #61310

Thank you @soyuka.

Am I right there is currently no way to specify which target class a collection item should be mapped to, in case when there are multiple mapped classes? So even if I make my own MapCollection transformer, the callback signature __invoke(mixed $value, object $source, ?object $target) is missing any mapping info about the property, so I'm unable to determine the mapped type.

in case when there are multiple mapped classes

This is not supported, this transformer only transforms to a collection of a single type. You can create your own transform and gather type at runtime but this is not optimal. Please open a new issue and detail your usage.

@soyuka The problem isn't about a mixed collection type but rather multiple possible collections of a single type. Tried to explain the problem I'm having in more detail:
#61957 (comment)