Remove cached value of reverse side of 020 relation when updating attname.

[Josh]

Repro:

from django.db import models


class AppUser(models.Model):
    pass


class Profile(models.Model):
    user = models.OneToOneField(
        to=AppUser,
        on_delete=models.PROTECT,
    )


a = AppUser.objects.create()
b = AppUser.objects.create()
Profile.objects.create(user=a)

b.profile # Errors, since it has not been created yet

a.profile.user_id = b.id
a.profile.save()

# Works!
Profile.objects.get(user=b)
# <Profile: Profile object (1)>

b.profile # Errors, since it uses the cached value

I can fix it by using a.profile.user = b instead of a.profile.user_id = b.id which does the cache invalidation as expected. Or using b.refresh_from_db() after the save() call. However I think it makes sense to fix this within the ORM because it's a subtle bug that can cause issues.
This is a contrived example but the real use-case was cloning a model instance. And instead of erroring it just kept returning the cached value

[Mariusz Felisiak]

Thanks for this ticket, I can see inconsistency but I'm not sure if it's feasible. Tentatively accepting for investigation.

The main difference is that profile.user = b uses one-to-one descriptors and has access to both side instances, on the other hand profile.user_id = b.id uses a deferred attribute.

This is a contrived example but the real use-case was cloning a model instance.

This is probably fixed by #31863.