Fix UpdateModelMixin to work when no queryset is defined is defined on the view

[browniebroke]

Description

It seems that the fix from #8043 isn't working in all cases, especially when the users aren't setting the queryset attribute, or overriding the get_queryset() method, and instead opt to override the get_object method.

Attempt to fix this edge case by delaying the call of get_queryset, to be only if the instance that was just updated had some prefetched relations.

Fix #9306

[browniebroke]

When only get_object is defined, I assume that folks are unlikely to have advanced prefetches in their method (although it's very much possible).

If some things were prefetched, we assume that there is a get_queryset method.

[yuekui]

Yeah this should fix the case that if only get_object() was defined, looks good to me

[mrzorn]

This is still expecting get_queryset or queryset to be defined and will raise an AssertError if it is not, which means it will still not be backwards compatabile with 3.14 correct?

[yuekui]

How is it possible that self has '_prefetched_objects_cache' defined without defining queryset or get_queryset?

[browniebroke]

That's what I meant by my comment earlier, one might do:

class UserRetrieveWithoutQuerySet(generics.RetrieveUpdateAPIView):
    serializer_class = UserSerializer

    def get_object(self):
        return User.objects.prefetch_related('groups').get(pk=self.kwargs['pk'])

In such case, I would argue that it's reasonable to require users to override the get_queryset method... The error message says exactly that. Would probably need to be documented better, though.

[yuekui]

Probably just an extra if statement or try catch would solve all these cases.

[auvipy]

should we revert the actual implementation as breaking change? and then come with a better fix? or in this case we can just go with this? #9327

[browniebroke]

Going to close this as it becoming clear that we should revert. I was only trying to help get a stable 3.15.x out, which a revert achieves...