prefetch_related_objects evaluates queryset when checking for invalid prefetch ordering.

[Iain P. Watts]

When encountering a prefetch it has already seen and checking whether to throw the "lookup was already seen with a different queryset" ValueError, prefetch_related_objects will inadvertently evaluate lookup.queryset. For example:

foo = Foo.objects.create()
prefetches = [
    'bar',
    Prefetch('bar', queryset=Bar.objects.filter(my_field=True))
]
prefetch_related_objects([foo], *prefetches)

Right before it throws the ValueError, the above code will execute the full unfiltered query Bar.objects.filter(my_field=True). This may match a very large number of rows (or a whole table), since it is not restricted to Bars related to the specific foo. In our case this led to the query timing out and the page returning a 504 without the code ever actually reaching the ValueError.

[Iain P. Watts]

​https://github.com/django/django/pull/11369

[Mariusz Felisiak <felisiak.mariusz@…>]

In 59ffafa1:

Fixed #30482 -- Prevented unnecessary evaluation of lookup.queryset in prefetch_related_objects().