Prefetch() assumes that a given queryset has an _iterable_class.

[Keryn Knight]

Problem reported on IRC, where the exception was essentially obscuring from the user why they can't proceed...

​The __init__ method for Prefetch tests whether the ._iterable_class is a subclass of ModelIterable in an effort to provide a runtime exception about trying to use values() (which is itself an oddly specific error message, because doesn't values_list() also return a non-ModelIterable?)

But there's at least one ostensible queryset type which one might try to use, which doesn't implement the same private API behind it's __iter__: RawQuerySet (as returned by MyObject.objects.raw(...))

Attempting to use Prefetch("x", queryset=MyObject.objects.raw(...)) thus raises an AttributeError when attempting to check whether it should raise a ValueError about values() usage.

I think it should actually test for hasattr(queryset, "_iterable_class") and raise a TypeError or ValueError about that, before trying to proceed with testing whether a specific private attribute is the correct type.

[Claude Paroz]

Regression in [7ec330eeb96d0]

[Mariusz Felisiak <felisiak.mariusz@…>]

In 4540842b:

Fixed #31044 -- Errored nicely when using Prefetch with a raw() queryset.

[Mariusz Felisiak <felisiak.mariusz@…>]

In d650527:

Refs #31044 -- Fixed error message when using Prefetch with a values_list() queryset.