id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
35779	ORM to avoid deferring the reference_id of a prefetch	Thiago Bellini Ribeiro	GunSliger00007	"Suppose that I have a `User` and an `Email` models, and they are related through `Email.user_id`

If I query `User` like this:

{{{
User.objects.prefetch_related(""email_set"", Email.objects.only(""email""))
}}}

I'll see n+1 issues either way, as `user_id` was deferred, and the ORM will need to refetch the object from the database.

Although the fix for this is easy, just changing this to:

{{{
User.objects.prefetch_related(""email_set"", Email.objects.only(""user_id"", ""email""))
}}}

It can catch some users off-guard, as it is not so obvious that the ORM will not do ""the correct thing"".

Based on that, I was wondering if maybe the ORM could force the reference_id of a relation to not be deferred when it is used in a `Prefetch` object, the same way it already does with `pk`. It makes sense to me at least.

---

On a side note, this caused some unexpected behavior at work today, as we were fixing `RemovedInDjango50Warnings` and the simple addition of `chunk_size` to an `.iterator()` with a `Prefetch` caused some `Model.DoesNotExist` issues.

After digging for a while I found out about this issue, which means our `prefetch_related` was being thrown into the trash, but also because the iteration was taking some time to happen (very large table), some related objects got deleted in the meantime, and when the ORM tried to `refresh_from_db` to get the related id, it failed with that error."	Uncategorized	closed	Database layer (models, ORM)	4.2	Normal	duplicate		Thiago Bellini Ribeiro	Unreviewed	0	0	0	0	0	0