Retrieves next or previous object by publish date. We implement
        our own version instead of Django's so we can hook into the
        published manager and concrete subclasses.

def _get_next_or_previous_by_publish_date(self, is_next, **kwargs):
        """
        Retrieves next or previous object by publish date. We implement
        our own version instead of Django's so we can hook into the
        published manager and concrete subclasses.
        """
        arg = "publish_date__gt" if is_next else "publish_date__lt"
        order = "publish_date" if is_next else "-publish_date"
        lookup = {arg: self.publish_date}
        concrete_model = base_concrete_model(Displayable, self)
        try:
            queryset = concrete_model.objects.published
        except AttributeError:
            queryset = concrete_model.objects.all
        try:
            return queryset(**kwargs).filter(**lookup).order_by(order)[0]
        except IndexError:
            pass