Support compound unique in upsert

[Nisgrak]

Describe the bug
The new upsert() fails if you try to upsert with a compound index

Stack trace

Error: Unique property value required for upsert, provide one of: id

To Reproduce
Steps to reproduce the behavior:
https://stackblitz.com/edit/mikro-orm-esm-repro-y96acp?file=basic.test.ts

Expected behavior
Generate an insert on conflict 'my_autogenerate_compound_index'

[B4nan]

Yes, this is not supported.

Funny how an explicit validation about something not being supported yields bug reports :D

[Nisgrak]

Yeah, maybe "bug" wasn't the aproppied word...

Anyway I see that Postgre and SQLite allow to specify what index have to conflict, maybe we can specify in the upsert()?

orm.em.upsert(User, { ... }, { index: "my_compound_index" } 

MariaDB doesn't seem to allow that.

[B4nan]

This is not about indexes, the problem is comparing complex PKs, that is the reason why this is disallowed, as you can't simply compare things like a.id === b.id, neither you can build a map of pk <-> entity easily.

TBH I was just lazy implementing this, the em.upsert turned to be very complex already even without that...

If you wanna take a look, its this part of codebase: https://github.com/mikro-orm/mikro-orm/blob/master/packages/core/src/EntityManager.ts#L523

This would have to additionally check composite unique constraints - those live on the entity metadata (meta.uniques), not on property level. I guess it won't be that hard to support, it was just too much to do in a single run.