Incorrect behavior of QuerySet.prefetch_related() in some circumstances during multilevel data prefetching

[Alex Sichkar]

In some circumstances prefetched data might be overwritten.

IMHO this ticket is related to the issue:
https://code.djangoproject.com/ticket/24873

If I revert changes from this pull request:
​https://github.com/django/django/pull/4723
there is no such problem.

Prepared sample project with test case to reproduce the issue.

[Tim Graham]

I found that your test case was fixed in Django 2.0 by 379caf397ea41923278821085204c296f960e70e. Since a regression test wasn't added as part of that commit, I'll accept the ticket to do that. If you can adapt your test for tests/prefetch_related (using existing models as much as possible) that would be great.

[Paulo]

PR: ​https://github.com/django/django/pull/9995

The commit in https://code.djangoproject.com/changeset/379caf397ea41923278821085204c296f960e70e/ didn't fix this.
It just reversed the execution order of prefetch objects.

Changing the attached test from:

projects = Project.objects.prefetch_related(
    Prefetch('tasks', queryset=tasks),
    Prefetch('tasks', to_attr='some_attr_name'),
)

to:

projects = Project.objects.prefetch_related(
    Prefetch('tasks', to_attr='some_attr_name'),
    Prefetch('tasks', queryset=tasks),
)

Made it fail again.

[Tim Graham <timograham@…>]

In 6104875a:

Fixed #29230 -- Fixed nested prefetches that clash with descriptors.

[Tim Graham <timograham@…>]

In 25d4d846:

[2.1.x] Fixed #29230 -- Fixed nested prefetches that clash with descriptors.

Backport of 6104875a2cc797bbd1254aa61f22a9b03d652128 from master