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\

Answer 1

I have a simpler version of frnhr's answer, which actually filters on __isnull condition. (Django 1.4+):

from django.contrib.admin import SimpleListFilter

class NullListFilter(SimpleListFilter):
    def lookups(self, request, model_admin):
        return (
            ('1', 'Null', ),
            ('0', '!= Null', ),
        )

    def queryset(self, request, queryset):
        if self.value() in ('0', '1'):
            kwargs = { '{0}__isnull'.format(self.parameter_name) : self.value() == '1' }
            return queryset.filter(**kwargs)
        return queryset

Then also:

class StartNullListFilter(NullListFilter):
    title = u'Started'
    parameter_name = u'started'

and finally:

class SomeModelAdmin(admin.ModelAdmin):
    list_filter =  (StartNullListFilter, )

I personally don't like to trash my admin.py with dozens of classes, so I came up with such a helper function:

def null_filter(field, title_=None):
    class NullListFieldFilter(NullListFilter):
        parameter_name = field
        title = title_ or parameter_name
    return NullListFieldFilter

Which I can later apply as in:

class OtherModelAdmin(admin.ModelAdmin):
    list_filter =  (null_filter('somefield'), null_filter('ugly_field', _('Beautiful Name')), )