Make assertQSEqual error out if no ordering defined

[Anssi Kääriäinen]

An unordered queryset isn't guaranteed to return the values in any order. So, using assertQuerysetEqual comparing more than one value and an unordered qs with the ordered flag set is likely a programming error.

As seen from the linked patch this error is fairly common.

In practice databases usually return the values in insert order, so the error does not surface often. Still, this can be a source of heisenbugs as background database maintenance jobs can alter the ordering of the results.

The linked patch contains two commits, one for making assertQuerysetEqual fail if the ordering isn't well defined, and one to fix all the tests in Django where the ordering isn't well defined.

Patch here: ​https://github.com/akaariai/django/compare/assertqs_ordering

I guess we need a short mention in release notes about this?

[Claude Paroz]

It seems that assertQuerysetEqual is not tested (in regressiontests/test_utils). Could you add some basic tests?

[Anssi Kääriäinen]

I have updated the patch (at ​https://github.com/akaariai/django/compare/assertqs_ordering). It now contains docs, tests for assertQuerysetEqual and in addition the assertQSEqual raises ValueError on undefined order instead of doing a self.fail(). The last change is from discussions at IRC.

I think this is now ready for commit. Review still welcome (especially docs review).

[Claude Paroz]

As far as code is concerned, this is indeed RFC. I've seen a typo ("is seens as"), but I don't count for a language reviewer :-)

[Anssi Kääriäinen]

I bet the docs changes contain a couple of things which would make my English teacher cry...

Still, I think I will commit this one with the typo fix, I have too many tickets in "minor polish needed, else ready" state...

[Anssi Kääriäinen <akaariai@…>]

In 088d3bc2f84b6b68fee7e5de053b58049bd110e7:

Fixed #19462 -- Made assertQuerysetEqual detect undefined ordering

If there are more than one values to compare against and the qs isn't
ordered then assertQuerysetEqual will raise a ValueError.