Support prefetch_related() with Queryset.aiterator()

[Barry Johnson]

Asynchronously iterating through a QuerySet that contains a prefetch_related definition is not supported in the alpha release of 4.2, but would be a quite useful feature.

If the iterator is chunked, related objects could be prefetched in a manner much like within QuerySet.iterator(), as long as the call to prefetch_related_objects is wrapped by sync_to_async().

[Barry Johnson]

Added PR ​https://github.com/django/django/pull/16543

(There are two commits -- the second one simply changes the order of an import statement. This GitHub newbie has been unable to find the "Squash and Merge" button anywhere on the GitHub user interface, nor find any documentation that says how to reach that button.)

[Simon Charette]

Thanks for the patch, I left some comments about on the PR.

[john-parton]

Open pull request here ​https://github.com/django/django/pull/17118

I have one basic test and the minimal change to documentation there.

[john-parton]

Updated my pull request

[Mariusz Felisiak <felisiak.mariusz@…>]

In fff1473:

Fixed #34331 -- Added QuerySet.aiterator() support for prefetch_related().

[nessita <124304+nessita@…>]

In 36b7024:

Fixed #35392, Refs #34331 -- Allowed importing aprefetch_related_objects from django.db.models.

Thanks James Ostrander for the report and fix, and to Tim Graham and
Mariusz Felisiak for the reviews.

[Natalia <124304+nessita@…>]

In ea81c9d:

[5.0.x] Fixed #35392, Refs #34331 -- Allowed importing aprefetch_related_objects from django.db.models.

Thanks James Ostrander for the report and fix, and to Tim Graham and
Mariusz Felisiak for the reviews.

Backport of 36b7024b7fc881b319a2b67876e4f72ba680d6a8 from main