Tweak formset to remove extraneous query

[Travis Swicegood]

A ModelFormset.save runs a SELECT due to its reliance on the if not self.get_queryset(): line. Immediately after that call, it loops over self.initial_forms and returns values set it that. In the instance where get_queryset() is empty, self.initial_forms is also empty as far as I can tell.

This patch changes the if not self.get_queryset() to an if not self.initial_forms so the extra select is not run. I've added tests that verify the behavior and have run the entire test suite. Everything continues to pass so I believe this is a safe patch to apply, but it definitely needs a second opinion of someone more familiar with model formsets than I.

[Travis Swicegood]

Original posting: ​https://github.com/django/django/pull/109

[Claude Paroz]

Looks fine. However I would place the test in model_formsets_regress (class FormsetTests), where an existing model can also be reused.

[Travis Swicegood]

Tweak based on feedback

[Travis Swicegood]

@claudep: Adjusted. All tests continue to pass and the patch is smaller.

[Jannis Leidel]

In [17434]:

Fixed #17594 -- Stopped ModelFormset.save from running a SELECT query by relying on the fact that the initial form is already set. Thanks, tswicegood.