Fixed #30355 -- Doc'd interaction between custom managers and prefetch_related().

[Akash-Kumar-Sen]

ticket #30355

[nessita]

Thank you @Akash-Kumar-Sen for this work. I have squashed commits, tweaked the text a little bit and changed the display to be an admonition:

I'll leave this PR here for a few days to see if other reviewers prefer other wording, otherwise I'll land before the week ends.

[smithdc1]

Thank you @Akash-Kumar-Sen for picking this up and pushing it forward. 👍

I'm slightly nervous this sounds like we're just documenting a bug / limitation of Django, but I'm fairly sure this isn't the case. I'm left with more questions than answers.

Could we add a little more to explain either why this is the correct behaviour or what the other means are to work around this are? (comments on ticket indicate there are, but it's not clear to me what that could be).

Is it possible to write something like?

Using prefetch_related uses the default manager because ... therefore use ... instead.

[felixxm]

I'm slightly nervous this sounds like we're just documenting a bug / limitation of Django, but I'm fairly sure this isn't the case. I'm left with more questions than answers.

I have the same feelings. The current behavior is what it is. I wouldn't document it as unexpected, maybe:

``QuerySet.prefetch_related()`` on a reverse relation always uses the default manager not a custom one.  ...

[Akash-Kumar-Sen]

@smithdc1 The possible explanation here is that queryset generated with prefetch does not modify the queryset or make a DB query when we are fetching the reviews, this is the sole purpose of prefetch, so if we try to use a custom manager with prefetch that will violate the purpose of using prefetch, hitting 1 extra query every time we try to fetch the reviews.
I will update the docs with this explanation for better understanding along with the alternative way using from django.db.models import Prefetch.

Attaching the testcase for better understanding: https://github.com/Akash-Kumar-Sen/django/blob/ticket_30355_3/tests/dummy_tests/tests.py

[felixxm]

@Akash-Kumar-Sen Thanks 👍 I pushed small formatting edits.