RelatedManager.get_prefetch_queryset returns "wrong" cache_name

[Mike Hansen]

Currently, RelatedManager.get_prefetch_queryset returns self.field.related_query_name() as the cache_name. In the case where no related_name has been set on the ForeignKey, then this does not match with the through_attr used by get_prefetcher.

Using the models in tests/prefetch_related/models.py,

BookWithYear.objects.prefetch_related('bookreview_set')

will use a through_attr of "bookreview_set", but "bookreview" is what will be placed in _prefetched_objects_cache.

I think related_manager.field.remote_field.get_accessor_name() should be used instead.

​https://github.com/django/django/pull/9259 is a pull request which fixes this issue

[Mike Hansen]

A pull request for this can be found at ​https://github.com/django/django/pull/9259

I wasn't sure on the best location for these tests so any guidance would be helpful.

[Tim Graham <timograham@…>]

In 514b2c98:

Fixed #28723 -- Fixed RelatedManager's prefetch_related() cache name.

[Mike Hansen]

Thanks Tim!

[Jon Dufresne]

I believe this change is causing a regression as the _remove_prefetched_objects() method was not updated to use the new cache name. See line:

​https://github.com/django/django/blob/514b2c989a948e3c59bda0da0c9427acf643cf5b/django/db/models/fields/related_descriptors.py#L578

I've created a ​PR to fix it.

[Tim Graham <timograham@…>]

In 8b218783:

Refs #28723 -- Fixed stale prefetch_related cache after add/remove.

Regression in 514b2c989a948e3c59bda0da0c9427acf643cf5b.