Hi Marc, agreed this looks like awesome to have.
Having it into the DSL is for sure a way, but I'm wondering if it could be defined on the indexing metadata? we could produce the join query transparently based on the field names.
I'll flag it as 5.1: we have many things on the roadmap already, and I don't think we'll be able to make it earlier. I'd rather have a quick 5.0 than release in ages, but we can start thinking about this in the scope of the internal refactorings.