models.py

from django.db import models, connection
from django.contrib.contenttypes.models import ContentType
from django.contrib.auth.models import User

class VoteManager(models.Manager):
    def get_score(self, obj):
        """
        Get the total score for ``obj`` and the number of votes it's received.
        """
        query = """
SELECT SUM(vote), COUNT(*) FROM %s
WHERE content_type_id = %%s
  AND object_id = %%s""" % self.model._meta.db_table
        ctype = ContentType.objects.get_for_model(obj)
        cursor = connection.cursor()
        cursor.execute(query, [ctype.id, obj.id])
        result = cursor.fetchall()[0]
        return {'score': result[0] or 0, 'num_votes': result[1]}

    def record_vote(self, obj, user, vote):
        """
        Record a user's vote on a given object. Only allows a given user
        to vote once, though that vote may be changed.

        A zero vote indicates that any existing vote should be removed.
        """
        if vote not in (+1, 0, -1):
            raise ValueError('Invalid vote (must be +1/0/-1)')
        ctype = ContentType.objects.get_for_model(obj)
        try:
            v = self.get(user=user, content_type=ctype, object_id=obj.id)
            if vote == 0:
                v.delete()
            else:
                v.vote = vote
                v.save()
        except Vote.DoesNotExist:
            if vote != 0:
                self.create(user=user, content_type=ctype,
                            object_id=obj.id, vote=vote)

    def get_top(self, Model, limit=10, reversed=False):
        """
        Get the top N scored objects for a given model.

        Yields (object, score) tuples.
        """
        ctype = ContentType.objects.get_for_model(Model)
        query = """
SELECT object_id, SUM(vote) AS score FROM %s
WHERE content_type_id = %%s
GROUP BY object_id""" % self.model._meta.db_table
        if reversed:
            query += 'HAVING SUM(vote) < 0 ORDER BY SUM(vote) ASC LIMIT %s'
        else:
            query += 'HAVING SUM(vote) > 0 ORDER BY SUM(vote) DESC LIMIT %s'
        cursor = connection.cursor()
        cursor.execute(query, [ctype.id, limit])
        results = cursor.fetchall()

        # Use in_bulk() to avoid O(limit) db hits.
        objects = Model.objects.in_bulk([id for id, score in results])

        # Yield each object, score pair. Because of the lazy nature of generic
        # relations, missing objects are silently ignored.
        for id, score in results:
            if id in objects:
                yield objects[id], score

    def get_bottom(self, Model, limit=10):
        """
        Get the bottom (i.e. most negative) N scored objects for a given model.

        Yields (object, score) tuples.
        """
        return self.get_top(Model, limit, True)

    def get_for_user(self, obj, user):
        """
        Get the vote made on the given object by the given user, or
        ``None`` if no matching vote exists.
        """
        if not user.is_authenticated():
            return None
        ctype = ContentType.objects.get_for_model(obj)
        try:
            vote = self.get(content_type=ctype, object_id=obj.id, user=user)
        except Vote.DoesNotExist:
            vote = None
        return vote

    def get_for_user_in_bulk(self, objects, user):
        """
        Get a dictionary mapping object ids to votes made by the given
        user on the corresponding objects.
        """
        vote_dict = {}
        if len(objects) > 0:
            ctype = ContentType.objects.get_for_model(objects[0])
            votes = list(self.filter(content_type__pk=ctype.id,
                                     object_id__in=[obj.id for obj in objects],
                                     user__pk=user.id))
            vote_dict = dict([(vote.object_id, vote) for vote in votes])
        return vote_dict

SCORES = (
    ('+1', +1),
    ('-1', -1),
)

class Vote(models.Model):
    user = models.ForeignKey(User)
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    object = models.GenericForeignKey('content_type', 'object_id')
    vote = models.SmallIntegerField(choices=SCORES)

    objects = VoteManager()

    class Meta:
        db_table = 'votes'
        verbose_name = 'Vote'
        verbose_name_plural = 'Votes'
        # Enforce one vote per user per object
        unique_together = (('user', 'content_type', 'object_id'),)

    class Admin:
        pass

    def __str__(self):
        return '%s: %s on %s' % (self.user, self.vote, self.object)

    def is_upvote(self):
        return self.vote == 1

    def is_downvote(self):
        return self.vote == -1