Support accessing the current filtered queryset from within SimpleListFilter.lookups()

[Simon Willison]

It would be really useful if code running inside a custom SimpleListFilter.lookups() method could access the filtered queryset for the current ModelAdmin instance, based on the current request.

Most significantly, this would allow admin filters to display counts for the number of items that would be returned if they were selected - for a classic faceted browse interface.

Sadly this isn't possible at the moment. I wrote about this in more detail here: ​https://til.simonwillison.net/django/almost-facet-counts-django-admin - but the short version is that the following:

class StateCountFilter(admin.SimpleListFilter):
    # ...
    def lookups(self, request, model_admin):
        changelist = model_admin.get_changelist_instance(request)
        qs = changelist.get_queryset(request)
        states_and_counts = qs.values_list(
            "state__abbreviation", "state__name"
        ).annotate(n = Count('state__abbreviation'))

Raises a RecursionError because the model_admin.get_changelist_instance(request) method itself triggers a call to this .lookups() method.

[Mariusz Felisiak]

Tentatively accepted, it looks reasonable but I'm not sure if it's doable with the current objects relations. Would you like to work on a patch?

[Simon Willison]

I'd love to, but I'm a bit stuck on where to begin with this.

Like you, I'm skeptical that this is a simple bug fix. I think there are two options here:

• A medium-sized change to the architecture of the admin, such that .lookups() no longer gets called when the ChangeList is constructed. I'm not at all sure how much work this would be or if it could be implemented without breaking existing custom code.
• A new, optional mechanism for people who want to use model_admin.get_changelist_instance(request) inside their .lookups() method - potentially even defining a new .lazy_options(...) method that people can implement which deliberately doesn't get called until later on, to avoid this RecursionError

I'd like it if there was a third option, "just rewrite things in an invisible way such that this works" - but my hunch is that's not possible. And the two options I listed above both warrant some discussion with people who are more familiar with the internals of the admin than I am!

[Sarah Boyce]

I was wondering if this is being worked on or if I could try to pick it up, happy to work on it in a group as well

[Carlton Gibson]

Hi Sarah — you're very welcome to pick it up. 🙂

I'd suggest an initial time-boxed investigation to work out what the state of play is.

At that point you'll be the world expert™ on this issue, so either you'll have an idea or you'll want input. If you open a PR at that point (even just adding comments in the second case) to explain what's going on, it's easier for others to get up to speed and input.

[Shreya Bamne]

I would like to try working on this ticket. I am not sure if someone is already working on this. (Claiming the ticket for now. I am happy to collaborate if someone is already working on this ticket. :))

[Shreya Bamne]

I have added comments in few files which would help to understand the issue. I am not sure what next steps are to fix it and I am looking for some input to begin working on this ticket. I hope the comments are helpful.

[Sarah Boyce]

​https://github.com/django/django/pull/16495

[Carlton Gibson]

The PR here looks promising. With a ​simple test project it correctly allows faceted (with counts) filtering.

• I think the documentation needs to spell out a bit more clearly what's going on (and the interrelation between multiple filters)
• Is the API OK...? — It's a bit clunky having to define each filter with the additional value added in the lookup label... — could we maybe make this work for all filters — especially the autogenerated ones from just a field name — perhaps with a query string toggle to turn faceting on for a given change list. (?..&factets=1) 🤔

[Sarah Boyce]

Second point is in the PR now, I have tried to improve the documentation, I think I would like to add a picture (as it will illustrate it better) but I'm not sure where the images are coming from - is there a project that is used for the documentation images?

[Mariusz Felisiak]

See ​comment.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 868e2fcd:

Fixed #32539 -- Added toggleable facet filters to ModelAdmin.

Thanks Carlton Gibson, Simon Willison, David Smith, and Mariusz
Felisiak for reviews.

[GitHub <noreply@…>]

In 2660bc1:

Refs #32539 -- Fixed facet filter tests on Oracle.

Follow up to 868e2fcddae6720d5713924a785339d1665f1bb9.

[GitHub <noreply@…>]

In cffcf0e:

Refs #32539 -- Fixed hide counts icon for RTL languages.

Bug in 868e2fcddae6720d5713924a785339d1665f1bb9.