base.py

    def _get_next_or_previous_by_FIELD(self, field, is_next, **kwargs):
        if not self.pk:
            raise ValueError("get_next/get_previous cannot be used on unsaved objects.")
        op = is_next and 'gt' or 'lt'
        order = not is_next and '-' or ''
        param = smart_str(getattr(self, field.attname))
        q = Q(**{'%s__%s' % (field.name, op): param})
        q = q|Q(**{field.name: param, 'pk__%s' % op: self.pk})
        qs = self.__class__._default_manager.using(self._state.db).filter(**kwargs).filter(q).order_by('%s%s' % (order, field.name), '%spk' % order)
        try:
            return qs[0]
        except IndexError:
            raise self.DoesNotExist("%s matching query does not exist." % self.__class__._meta.object_name)