Filtering Django Admin by Null/Is Not Null

Question

I have a simple Django model like:

class Person(models.Model):
    referrer = models.ForeignKey('self', null=True)
    ...

In this model's ModelAdmin, how would I allow it to be filtered by whether or not referrer is null? By default, adding referrer to list_filter causes a dropdown to be shown that lists every person record, which may be in the hundreds of thousands, effectively preventing the page from loading. Even if it loads, I still can't filter by the criteria I want.

i.e. How would I modify this so that the dropdown only lists "All", "Null", or "Not Null" choices?

I've seen some posts that claim to accomplish something similar using custom FilterSpec subclasses, but none of them explain how to use them. The few I've seen appear to apply to all fields in all models, which I wouldn't want. Moreover, there's zero documentation for FilterSpec, which makes me nervous, because I don't want to invest in a lot of custom code tied to some transient internal class that might disappear by the next release.

Answer 1

I ended up using a mixture of the top solution here, along with this snippet.

However, I had to tweak the snippet slightly, dropping the field type restriction and adding the new field_path, recently added in 1.3.

from django.contrib.admin.filterspecs import FilterSpec
from django.db import models
from django.utils.safestring import mark_safe
from django.utils.translation import ugettext as _

class NullFilterSpec(FilterSpec):
    #fields = (models.CharField, models.IntegerField, models.FileField)

    @classmethod
    def test(cls, field):
        #return field.null and isinstance(field, cls.fields) and not field._choices
        return field.null and not field._choices
    #test = classmethod(test)

    def __init__(self, f, request, params, model, model_admin, field_path=None):
        super(NullFilterSpec, self).__init__(f, request, params, model, model_admin, field_path)
        self.lookup_kwarg = '%s__isnull' % f.name
        self.lookup_val = request.GET.get(self.lookup_kwarg, None)

    def choices(self, cl):
        # bool(v) must be False for IS NOT NULL and True for IS NULL, but can only be a string
        for k, v in ((_('All'), None), (_('Has value'), ''), (_('Omitted'), '1')):
            yield {
                'selected' : self.lookup_val == v,
                'query_string' : cl.get_query_string({self.lookup_kwarg : v}),
                'display' : k
            }

# Here, we insert the new FilterSpec at the first position, to be sure
# it gets picked up before any other
FilterSpec.filter_specs.insert(0,
    # If the field has a `profilecountry_filter` attribute set to True
    # the this FilterSpec will be used
    (lambda f: getattr(f, 'isnull_filter', False), NullFilterSpec)
)