Django: Annotate based on an annotation - django

Question

Let's say I'm using Django to manage a database about athletes:
class Player(models.Model):
name = models.CharField()
weight = models.DecimalField()
team = models.ForeignKey('Team')
class Team(models.Model):
name = models.CharField()
sport = models.ForeignKey('Sport')
class Sport(models.Model):
name = models.CharField()
Let's say I wanted to compute the average weight of the players on each team. I think I'd do:
Team.objects.annotate(avg_weight=Avg(player__weight))
But now say that I want to compute the variance of team weights within each sport. Is there a way to do that using the Django ORM? How about using the extra() method on a QuerySet? Any advice is much appreciated.

Answer 1

you can use query like this :
class SumSubquery(Subquery):
template = "(SELECT SUM(`%(field)s`) From (%(subquery)s _sum))"
output_field = models.Floatfield()
def as_sql(self, compiler, connection, template=None, **extra_context):
connection.ops.check_expression_support(self)
template_params = {**self.extra, **extra_context}
template_params['subquery'], sql_params = self.queryset.query.get_compiler(connection=connection).as_sql()
template_params["field"] = list(self.queryset.query.annontation_select_mask)[0]
sql = template % template_params
return sql, sql_params
Team.objects.all().values("sport__name").annotate(variance=SumSubquery(Player.objects.filter(team__sport_id=OuterRef("sport_id")).annotate(sum_pow=ExpressionWrapper((Avg("team__players__weight") - F("weight"))**2,output_field=models.Floatfield())).values("sum_pow"))/(Count("players", output_field=models.FloatField())-1))
and add related name to model like this:
class Player(models.Model):
name = models.CharField()
weight = models.DecimalField()
team = models.ForeignKey('Team', related_name="players")

Answer 2

I'm going to assume (perhaps incorrectly) that you mean by 'variance' the difference between maximum and minimum weights. If so, you can generate more than one aggregate with a single query, like so:
from django.db.models import Avg, Max, Min
Team.objects.aggregate(Avg('player__weight'), Max('player__weight'), Min('player__weight'))
This is taken from the django docs on generating aggregation over a queryset.