Filter Count Mixin for Django's admin

facets.py

from django.contrib import admin
from django.db.models import Count, F

class FilterCountMixin(admin.SimpleListFilter):
    def __init__(self, request, params, model, model_admin):
        # Store the request so we can reapply all the filters properly later
        self.request = request
        super().__init__(request, params, model, model_admin)
        
    def get_filter_counts_parameter_name(self):
        return self.parameter_name

    def get_filter_counts_queryset(self, qs):
        return qs

    def choices(self, changelist):
        choices = super().choices(changelist)

        qs = changelist.root_queryset

        # To get the counts for *this* filter, we have to get the final queryset for the
        # changelist with every filter applied *except* this one, then we can do our
        # aggregations from that point.
        for filter_spec in changelist.filter_specs:
            if filter_spec is not self:
                new_qs = filter_spec.queryset(self.request, qs)
                if new_qs is not None:
                    qs = new_qs

        counts = (
            self.get_filter_counts_queryset(qs)
            .annotate(count_lookup_field=F(self.get_filter_counts_parameter_name()))
            .values("count_lookup_field")
            .annotate(total=Count("id"))
        )

        total = sum(x["total"] for x in counts)

        lookup_choices = self.lookup_choices.copy()
        lookup_choices.insert(0, None)

        for (spec, choice) in zip(lookup_choices, choices):  # type: ignore
            if spec is None:
                choice["display"] = f'{choice["display"]} ({total})'
            else:
                count = next(
                    (x["total"] for x in counts if x["count_lookup_field"] == spec[0]),
                    0,
                )
                choice["display"] = f'{choice["display"]} ({count})'
            yield choice