'Return choices with a default blank choices included, for use
as <select> choices for this field.
Analog of django.db.models.fields.Field.get_choices(), provided
initially for utilization by RelatedFieldListFilter.'

def get_choices(self, include_blank=True, blank_choice=BLANK_CHOICE_DASH):

    return ((blank_choice if include_blank else []) + [(x.pk, str(x)) for x in self.related_model._default_manager.all()])
