gistfile1.py

    # Django doesn't fire signals for parent Models.
    # https://code.djangoproject.com/ticket/9318
    def _save_table(self, raw=False, cls=None, force_insert=False,
            force_update=False, using=None, update_fields=None):

        if cls is not self.__class__:
            signals.pre_save.send(sender=cls, instance=self,
                raw=raw, using=using, update_fields=update_fields)

        updated = super(Node, self)._save_table(raw, cls, force_insert,
            force_update, using, update_fields)

        if cls is not self.__class__:
            signals.post_save.send(sender=cls, instance=self, created=(not updated),
                raw=raw, using=using, update_fields=update_fields)