Add ordering mixin for class based generic views

[Aymeric Augustin]

Currently, if the model used by by a date-based generic view doesn't have an ordering and pagination is enabled, results will be random. Defining an ordering on the model isn't totally satisfying because it has global effects, and it adds an overhead to every query involving the model.

Another workaround is to define queryset = MyModel.objects.order_by('-<date_field>') in every view. But CBVs are precisely about removing as much boilerplate as possible.

So here's a draft of an API for ordering:

class BaseDateListView(...):

    ordering = None

    def get_ordering(self):
        return self.get_date_field() if self.ordering is None else self.ordering
            
    def get_dated_queryset(self, **lookups):

    ...

    if not qs.ordered():
        qs = qs.order_by(self.get_ordering())

    ...

self.get_ordering() could return '-<date_field>' for the archive view for backwards compatibility.

This would be a better fix for #18354.

[Jannis Leidel]

I accept the problem in general, but don't see how this is limited to date based views. This is perfect for a mixin, so let's write a OrderingViewMixin or similar.

[Marc Tamlyn]

Would this functionality apply only to list views? This would certainly give a good advantage as one's instinct is to apply the get_queryset changes to every view with the same Model, but ordering is rather useless when we're simply doing a DetailView (and could impose a performance problem). So the code would perhaps live in BaseListView rather than MultipleObjectMixin.

That said, if this functionality is only hitting list views, it actually fits in with a larger subject area of search/ordering/filtering as defined by the user. In reality this is probably just one of the first hooks we might need to implement that kind of functionality well, but it may be worth considering the larger list-view-customisation issue.

[Aymeric Augustin]

The goals were clear to me when I was working on the date-based views, but unfortunately I've lost track of it since then...

I think it's just about implementing the API in the ticket description, documenting it, and taking advantage of it in the date-based views while preserving backwards-compat.

You're very welcome to take over on this ticket if you'd like to as it's pretty low on my priority list.

[anonymous]

I've added a PR for this here:

​https://github.com/django/django/pull/2097

Not sure if the approach is right - feedback welcome. I suspect the docs need improvement - more than happy to do that if the code is looking good.

[pjrharley]

Try assigning to myself while signed in instead...

[Tim Graham]

I left comments for improvement on PR. Please uncheck "Patch needs improvement" when you update it, thanks.

[pjrharley]

Patch updated. Thanks for the review.

[Tim Graham <timograham@…>]

In 2724cdbff635a40819f206411de23e9b14867a58:

Fixed #18355 -- Added ordering options to list based generic views.

Added MultipleObjectMixin.ordering and get_ordering().

Refs #21450.