Support prefetch_related() with Queryset.iterator()

[Taylor H]

I was surprised when I found out that prefetch_related calls are ignored when using Queryset.iterator. I noticed in the docs here ​https://docs.djangoproject.com/en/dev/ref/models/querysets/#iterator that it is because the "these two optimizations do not make sense together." That may have been true in the past, but it is definitely not the case now. The iterator allows chunking (by default 2000) and it would be very helpful to prefetch related for each chunk.

[Tim Graham]

The situation isn't completely straightforward. There's a discussion on ​django-developers. Accepting the ticket to do something.

[Raphael Kimmig]

I've created a proof of concept implementation ​https://github.com/django/django/pull/10707/

I think prefetching is probably closer to what a user would expect from the api.

[Simon Charette]

Patch needs both documentation and a deprecation warning when self._prefetch_related_lookups and chunk_size is None. The idea is to eventually turn this deprecation warning into an error and never performs prefetching unless a chunk size is specified.

[Jacob Walls]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In edbf9302:

Fixed #29984 -- Added QuerySet.iterator() support for prefetching related objects.

Co-authored-by: Raphael Kimmig <raphael.kimmig@…>
Co-authored-by: Simon Charette <charette.s@…>

[Mariusz Felisiak <felisiak.mariusz@…>]

In 1391356:

Refs #29984 -- Made QuerySet.iterator() without chunk_size raise ValueError after prefetch_related().

Per deprecation timeline.