假设我正在使用 Django 来管理有关运动员的数据库:
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()
假设我想计算每支球队球员的平均体重。我想我会这么做:
Team.objects.annotate(avg_weight=Avg(player__weight))
但现在说我想计算每项运动中团队权重的方差。有没有办法使用 Django ORM 来做到这一点?在 QuerySet 上使用
extra()
方法怎么样?非常感谢任何建议。
您可以使用这样的查询:
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))
并将相关名称添加到模型中,如下所示:
class Player(models.Model):
name = models.CharField()
weight = models.DecimalField()
team = models.ForeignKey('Team', related_name="players")
我将假设(可能是错误的)你所说的“方差”是指最大和最小权重之间的差异。如果是这样,您可以使用单个查询生成多个聚合,如下所示:
from django.db.models import Avg, Max, Min
Team.objects.aggregate(Avg('player__weight'), Max('player__weight'), Min('player__weight'))
这取自 django 文档关于通过查询集生成聚合。
您可以尝试在团队内部使用@property
类似:
class Team(models.Mode):
name = models.CharField()
sport = models.ForeignKey('Sport')
@property
def avg_weight(self):
list_of_weights = []
for player in Player.objects.filter(Team = self):
list_of_weights.append(player.weight)
return sum(list_of_weights) / len(list_of_weights)