Trouble with Django annotation with multiple ForeignKey references - django

Question

I'm struggling with annotations and haven't found examples that help me understand. Here are relevant parts of my models:
class Team(models.Model):
team_name = models.CharField(max_length=50)
class Match(models.Model):
match_time = models.DateTimeField()
team1 = models.ForeignKey(
Team, on_delete=models.CASCADE, related_name='match_team1')
team2 = models.ForeignKey(
Team, on_delete=models.CASCADE, related_name='match_team2')
team1_points = models.IntegerField(null=True)
team2_points = models.IntegerField(null=True)
What I'd like to end up with is an annotation on the Teams objects that would give me each team's total points. Sometimes, a team is match.team1 (so their points are in match.team1_points) and sometimes they are match.team2, with their points stored in match.team2_points.
This is as close as I've gotten, in maybe a hundred or so tries:
teams = Team.objects.annotate(total_points =
Value(
(Match.objects.filter(team1=21).aggregate(total=Sum(F('team1_points'))))['total'] or 0 +
(Match.objects.filter(team2=21).aggregate(total=Sum(F('team2_points'))))['total'] or 0,
output_field=IntegerField())
)
This works great, but (of course) annotates the total_points for the team with pk=21 to every team in the queryset. If there's a better approach for all this, I'd love to see it, but short of that, if you can show me how to turn those '21' values into a reference to the outer team's pk, I think that will work?
EDIT: I ended up using a combination of elyas' answers and annotating a raw SQL statement to solve my issues. I was not able to keep normal annotations from dropping non-unique scores from the queryset, but raw SQL seems to work.
Here's that raw annotation:
teams = Team.objects.raw('select id, sum(points) as total_points from (select team1_id as id, team1_points as points from leagueman_match union all select team2_id as id, team2_points as points from leagueman_match) group by id order by total_points desc;')

Answer 1

An alternative solution to annotating the QuerySet might be to make total_points a #property of the Team model (depending on the use case):
from django.db.models import Case, Q, Sum, When
class Team(models.Model):
team_name = models.CharField(max_length=50)
#property
def total_points(self):
return Match.objects.filter(Q(team1=self.id) | Q(team2=self.id)).aggregate(
total_points=Sum(Case(
When(team1=self.id, then='team1_points'),
When(team2=self.id, then='team2_points')
))
)['total_points']
The disadvantage is that it can't be used in subsequent QuerySet operations e.g. .values(), .order_by().
Django also has a #cached_property decorator which will cache the output of the attribute when it is first called.
Other solutions tried
Originally I thought you could leverage the reverse relations match_team1 and match_team2 from the Team model to generate a simple annotation:
teams = Team.objects.annotate(
total_points=(
Sum('match_team1__team1_points', distinct=True)
+ Sum('match_team2__team2_points', distinct=True)
)
)
Unfortunately this solution encounters difficulties in handling duplicates. The distinct=True argument eliminates the issue of points from the same match being summed more than once. But it introduces a different issue where different matches with the same points scored will be excluded.

Answer 2

Maybe I would advice to refactor your data model. Even if you find a solution for this specific problem, you may want to think little ahead.
This is a solution:
class Team(models.Model):
team_name = models.CharField(max_length=50)
class Match(models.Model):
match_time = models.DateTimeField()
team1 = models.ForeignKey(
Team, on_delete=models.CASCADE, related_name='match_team1')
team2 = models.ForeignKey(
Team, on_delete=models.CASCADE, related_name='match_team2')
class Points(models.Model):
match = models.ForeignKey(
Match, on_delete=models.CASCADE, related_name='match')
team = models.ForeignKey(
Team, on_delete=models.CASCADE, related_name='team')
points = models.IntegerField(null=True)
With this you can sum up easily the points of any team, and also filter it by matches.