In the admin sidebar, only show related objects that are in use by the objects shown in the list

admin_filters.py

from django.contrib import admin
from django.contrib.admin.utils import get_model_from_relation
from django.utils.encoding import smart_text


class LimitedRelatedFieldListFilter(admin.RelatedFieldListFilter):
    """
    Limit the filter choices in the admin sidebar to objects
    which are actually used by the objects shown in the list.

    Usage::

        list_filter = (
            ('fieldname', LimitedRelatedFieldListFilter),
        )
    """

    def __init__(self, field, request, params, model, model_admin, field_path):
        # NOTE:
        # Replaced the entire __init__ code,
        # to offer a get_queryset() hook that allows limiting the queryset.
        other_model = get_model_from_relation(field)
        if hasattr(field, 'rel'):
            rel_name = field.rel.get_related_field().name
        else:
            rel_name = other_model._meta.pk.name
        self.lookup_kwarg = '%s__%s__exact' % (field_path, rel_name)
        self.lookup_kwarg_isnull = '%s__isnull' % field_path
        self.lookup_val = request.GET.get(self.lookup_kwarg, None)
        self.lookup_val_isnull = request.GET.get(
            self.lookup_kwarg_isnull, None)

        self.lookup_choices = [
            (x._get_pk_val(), smart_text(x)) for x in self.get_queryset(field)
        ]
        admin.FieldListFilter.__init__(
            self, field, request, params, model, model_admin, field_path)

        if hasattr(field, 'verbose_name'):
            self.lookup_title = field.verbose_name
        else:
            self.lookup_title = other_model._meta.verbose_name
        self.title = self.lookup_title
        self.empty_value_display = model_admin.get_empty_value_display()

    def get_queryset(self, field):
        # Allow filtering the queryset!
        other_model = get_model_from_relation(field)
        return other_model._default_manager.complex_filter(field.rel.limit_choices_to)


class SortedLimitedRelatedFieldListFilter(LimitedRelatedFieldListFilter):
    """
    Override the standard RelatedField list filter, to sort the values.

    Usage::

        list_filter = (
            ('fieldname', SortedLimitedRelatedFieldListFilter),
        )
    """

    def __init__(self, *args, **kwargs):
        super(SortedLimitedRelatedFieldListFilter, self).__init__(*args, **kwargs)
        self.lookup_choices = sorted(self.lookup_choices, key=lambda a: a[1].lower())