Can I make list_filter in django admin to only show referenced ForeignKeys?

Question

I have a django application which has two models like this:

class MyModel(models.Model):
    name = models.CharField()
    country = models.ForeignKey('Country')

class Country(models.Model):
    code2 = models.CharField(max_length=2, primary_key=True)
    name = models.CharField()

The admin class for MyModel looks like this:

class MyModelAdmin(admin.ModelAdmin):
    list_display = ('name', 'country',)
    list_filter = ('country',)
admin.site.register(models.MyModel, MyModelAdmin)

The Country table contains ~250 countries. Only a handful of countries are actually referenced by some MyModel instance.

The problem is that the list filter in the django admin lists ALL countries in the filter panel. Listing all countries (and not just those that are referenced by an instance) pretty much defeats the purpose of having the list filter in this case.

Is there some to only display the countries referenced by MyModel as choices in the list filter? (I use Django 1.3.)

Answer 1

As of Django 1.8, there is a built in RelatedOnlyFieldListFilter, which you can use to show related countries.

class MyModelAdmin(admin.ModelAdmin):
    list_display = ('name', 'country',)
    list_filter = (
        ('country', admin.RelatedOnlyFieldListFilter),
    )

For Django 1.4-1.7, list_filter allows you to use a subclass of SimpleListFilter. It should be possible to create a simple list filter that lists the values you want.

If you can't upgrade from Django 1.3, you'd need to use the internal, and undocumented, FilterSpec api. The Stack Overflow question Custom Filter in Django Admin should point you in the right direction.