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

there is a simple way:

class RefererFilter(admin.SimpleListFilter):
    title = 'has referer'
    # Parameter for the filter that will be used in the URL query.
    parameter_name = 'referer__isnull'

    def lookups(self, request, model_admin):
        return (
            ('False', 'has referer'),
            ('True', 'has no referer'),
        )

    def queryset(self, request, queryset):
        if self.value() == 'False':
            return queryset.filter(referer__isnull=False)
        if self.value() == 'True':
            return queryset.filter(referer__isnull=True)

Then just used them in ModelAdmin:

class PersonAdmin(admin.ModelAdmin):
    list_filter =  (RefererFilter,)