gistfile1.py

class BaseModelAdmin(object):
    #...snip...
    def formfield_for_manytomany(self, db_field, request=None, **kwargs):
        """
        Get a form Field for a ManyToManyField.
        """
        # If it uses an intermediary model that isn't auto created, don't show
        # a field in admin.
        if not db_field.rel.through._meta.auto_created:
            return None
        db = kwargs.get('using')

        if db_field.name in self.raw_id_fields:
            kwargs['widget'] = widgets.ManyToManyRawIdWidget(db_field.rel, using=db)
            kwargs['help_text'] = ''
        elif db_field.name in (list(self.filter_vertical) + list(self.filter_horizontal)):
            kwargs['widget'] = widgets.FilteredSelectMultiple(db_field.verbose_name, (db_field.name in self.filter_vertical))

        return db_field.formfield(**kwargs)