Django Aggregate by QuerySet?

Question

I'm writing a league system and I want to display player rankings in each season sorted by points each player accumulated during that season.

So far I managed to do it with code similar to this:

class Player(models.Model):
    name = models.CharField(max_length=100)


class Season(models.Model):
    name = models.CharField(max_length=100)
    start_date = models.DateField()
    end_date = models.DateField()
    players = models.ManyToManyField(Player)

    def get_player_rank(self, player):
        return player.matchresult_set.filter(season=self).aggregate(points=Sum('points'))['points']

    def get_ranks(self):
        ranks = [(player, self.get_player_rank(player)) for player in self.players.all()]
        ranks.sort(key=lambda tup: tup[1])
        return ranks


class Match(models.Model):
    date = models.DateField()
    players = models.ManyToManyField(Player, through='MatchResult')
    season = models.ForeignKey(Season)


class MatchResult(models.Model):
    player = models.ForeignKey(Player)
    match = models.ForeignKey(Match)
    points = models.IntegerField()

I think the same could be achieved by a much simpler aggregation, but I just can't get the annotate() right.

I tried this but it just summed all points throughout the seasons:

class Season(models.Model):    
    def get_ranks(self):
        return self.players.annotate(points=Sum('matchresult__points')).order_by('-points')

What am I missing? I guess .extra() can be used if it would result in portable code.

Answer 1

This returns usable results:

Season.objects.values('name','match__matchresult__player__username').annotate(points=Sum('match__matchresult__points')).distinct()

I also needed to implement SumWithDefault to get rid of NULLs:

from django.db.models.sql.aggregates import Aggregate
from django.db.models import Aggregate as Ag

class SumWithDefaultSQL(Aggregate):
    def __init__(self, col, default=None, **extra):
        super(SumWithDefaultSQL, self).__init__(col, default=default, **extra)
        self.sql_function = 'SUM'
        if default is not None:
            self.sql_template = 'COALESCE(%(function)s(%(field)s), %(default)s)'


class SumWithDefault(Ag):
    name = 'Sum'

    def add_to_query(self, query, alias, col, source, is_summary):
        aggregate = SumWithDefaultSQL(col, source=source, is_summary=is_summary, **self.extra)
        query.aggregates[alias] = aggregate

final query:

Season.objects.values('name','match__matchresult__player__username').annotate(points=SumWithDefault('match__matchresult__points', default=0)).distinct().order_by('-points')

Answer 2

Django's ORM does not cover every use case.

You could store the aggregates in a separate model or fall back to raw SQL.