gistfile1.py

class PostSort(models.Model):
    """
    You must create the following index by hand:

        create index forums_postsort_sort on forums_postsort (thread_id, sort, value, depth);

    >>> post_ids = PostSort.objects.filter(thread_id=2, sort=0)\
                                   .order_by('value', 'depth')\
                                   .values_list('post_id', flat=True)[:25]
    """
    SORT_CHOICES = dict((
        (0, 'newest'),
        (1, 'oldest'),
    ))

    thread_id = models.PositiveIntegerField()
    post_id = models.PositiveIntegerField()
    depth = models.PositiveIntegerField(default=1)
    sort = models.PositiveIntegerField(choices=SORT_CHOICES.items())
    value = NumericArrayField()

    class Meta:
        unique_together = (('thread_id', 'post_id', 'sort'),)

    @classmethod
    def get_sort(cls, flat, sort, instance):
        try:
            ts = float(instance.date.strftime('%s.%f'))
        except ValueError:
            ts = 0
        if sort == 0: # oldest
            value = [ts]
        elif sort == 1: # newest
            value = [-ts]
        else:
            raise ValueError

        value.append(instance.parent_post_id or instance.pk)

        return value

    @classmethod
    def post_save(cls, instance, created=True, **kwargs):
        if not switches.for_forum('post_sort_paginator_index', instance.forum):
            return

        if not created:
            return

        for flat in (True, False):
            for sort in cls.SORT_CHOICES.keys():
                if instance.parent_post_id and not flat:
                    value = cls.objects.filter(
                        thread_id=instance.thread_id,
                        post_id=instance.parent_post_id,
                        sort=sort,
                    ).values_list('value', flat=True).get()
                else:
                    value = []

                this_value = cls.get_sort(flat, sort, instance)
                depth = 1
                if instance.parent_post_id:
                    depth += len(value) / len(this_value)

                cls.objects.create(
                    thread_id=instance.thread_id,
                    post_id=instance.pk,
                    sort=sort,
                    depth=depth,
                    value=value + this_value,
                )