Hacker News algorithm for django-voting sort order

Question

I'm working on an application using django-voting and have the sort order of the homepage items working using Eric Florenzano's custom VoteAwareManager technique:

models.py

class VoteAwareManager(models.Manager):
    """ Get top votes. hot = VoteAwareManager() """
    def _get_score_annotation(self):
        model_type = ContentType.objects.get_for_model(self.model)
        table_name = self.model._meta.db_table
        return self.extra(select={
            'score': 'SELECT COALESCE(SUM(vote),0) FROM %s WHERE content_type_id=%d AND object_id=%s.id' %
                (Vote._meta.db_table, int(model_type.id), table_name)
                }
        )

    def most_loved(self,):
        return self._get_score_annotation().order_by('-score')

    def most_hated(self):
        return self._get_score_annotation().order_by('score')

class Post(models.Model):
    """Post model"""
    title = models.CharField(_("title"), max_length=200, blank=False)
    slug = models.SlugField(_("slug"), blank=True)
    author = models.ForeignKey(User, related_name="added_posts")
    kind = models.CharField(max_length=1, choices=KIND, default=1)
    url = models.URLField(blank=True, null=True, help_text="The link URL", default='')
    content_markdown = models.TextField(_("Entry"), blank=True)
    content_html = models.TextField(blank=True, null=True, editable=False)
    status = models.IntegerField(_("status"), choices=STATUS_CHOICES, default=IS_PUBLIC)
    allow_comments = models.BooleanField(_("Allow Comments?"), blank=False, default=1)
    created_at = models.DateTimeField(_("created at"), default=datetime.now)
    updated_at = models.DateTimeField(_("updated at"))

    objects = models.Manager()
    hot = VoteAwareManager()

views.py

def homepage(request): 
"""Show top posts"""   
return object_list(request, 
    queryset=Post.hot.most_loved().filter(status=IS_PUBLIC),
    template_name='homepage.html',
    template_object_name='post',
    extra_context= {'profile': get_profiles}
)

I would now like to combine Hacker New's ranking algorithm with the code above so that older items get moved down in rank, but I am having trouble. I am not sure if the relevant code should go into the VoteAwareManager function, or the most_loved method, or elsewhere altogether.

The following is what I have tried:

1. Calculation in most_loved method: returns TypeError at / unsupported operand type(s) for -: 'QuerySet' and 'int' (when using a random timestamp just to see if I can get a result, eventually I need to figure out how to get the object timestamp, too — I am a beginning programmer):

def most_loved(self):
    totalscore = self._get_score_annotation()
    time_stamp = 20120920
    gravity = 1.8
    return (totalscore - 1) / pow((time_stamp+2), gravity)

2. Calculation in SQL: returns TemplateSyntaxError at / Caught DatabaseError while rendering: column "votes.time_stamp" must appear in the GROUP BY clause or be used in an aggregate function LINE 1: ...(SELECT COALESCE(SUM(vote),0 / (EXTRACT(HOUR FROM TIME_STAMP...:

class VoteAwareManager(models.Manager):
""" Get top votes. hot = VoteAwareManager() """
def _get_score_annotation(self):
    model_type = ContentType.objects.get_for_model(self.model)
    table_name = self.model._meta.db_table
    return self.extra(select={
        'score': 'SELECT COALESCE(SUM(vote),0 / (EXTRACT(HOUR FROM TIME_STAMP)+2 * 1.8)) FROM %s WHERE content_type_id=%d AND object_id=%s.id' % 
        (Vote._meta.db_table, int(model_type.id), table_name)
        }
    )

One option is attempt to change the voting system to use django-rangevoting, but I'd like to get this working with django-voting if possible. Any help much appreciated.

Answer 1

Not perfect (omits the -1 subtraction to negate user's own vote), but this seems to work well enough for now:

class VoteAwareManager(models.Manager):
""" Get recent top voted items (hacker news ranking algorythm, without the -1 for now since it breaks the calculation as all scores return 0.0)
    (p - 1) / (t + 2)^1.5
    where p = points and t = age in hours
"""
def _get_score_annotation(self):
    model_type = ContentType.objects.get_for_model(self.model)
    table_name = self.model._meta.db_table

    return self.extra(select={

        'score': 'SELECT COALESCE(SUM(vote / ((EXTRACT(EPOCH FROM current_timestamp - created_at)/3600)+2)^1.5),0) FROM %s WHERE content_type_id=%d AND object_id=%s.id' % (Vote._meta.db_table, int(model_type.id), table_name)

        })

def most_loved(self):        
    return self._get_score_annotation().order_by('-score',)