RelatedFieldListFilter vs RelatedOnlyFieldListFilter

RelatedFieldListFilter.txt

from the source:

class RelatedOnlyFieldListFilter(RelatedFieldListFilter):
  def field_choices(self, field, request, model_admin):
    pk_qs = model_admin.get_queryset(request).distinct().values_list('%s__pk' % self.field_path, flat=True)
    return field.get_choices(include_blank=False, limit_choices_to={'pk__in': pk_qs})
    
ex if:
class Author(models.Model):
  country = models.ForeignKey('Country')
  
RelatedFieldListFilter will show ALL countries.
RelatedOnlyFieldListFilter will only show countries that have at least 1 Author assigned to them

NOTE: (this took me some googling, wish it were more apparent in the docs)
  Filters will only show up in the side bar if there is more than one option.
  So, for the example above, if RelatedOnlyFieldListFilter were used, 
  there would need to be an author in at least two countries before the filter showed up in the sidebar.