如何通过Django中的单个查询器进行聚合? [英] How to aggregate over a single queryset in Django?
问题描述
简短描述:给定一个查询器 myQueryset
,如何选择 max(myfield)
没有实际检索所有行,并在python中执行 max
我能想到的最好的是 max([r [myfield] for r in myQueryset.values(myfield)]),如果有数百万行,这不是很好。 / p>
长描述:说我的Django应用程序,城市和国家中有两个模型。城市有一个国家的外键字段:
class国家(models.Model):
name = models.CharField (max_length = 256)
class城市(models.Model):
name = models.CharField(max_length = 256)
population = models.IntegerField()
country = models.ForeignKey(Country,related_name ='cities')
这意味着一个Country实例 .cities
可用。假设我现在想编写一个名为 highest_city_population
的方法返回最大城市的人口。来自LINQ的背景,我的本能就是尝试 myCountry.cities.max('population')
或类似的东西,但这是不可能的。 >
使用聚合(Django 1.1中的新增)。你这样使用:
>>>从django.db.models import Max
>>> City.objects.all()。aggregate(Max('population'))
{'population__max':28025000}
为了获得每个国家
的 City
的最高人口,我想你可以做这样的:
>>>从django.db.models import Max
>>> Country.objects.annotate(highest_city_population = Max('city__population'))
Short description: given a queryset myQueryset
, how do I select max("myfield")
without actually retrieving all rows and doing max
in python?
The best I can think of is max([r["myfield"] for r in myQueryset.values("myfield")])
, which isn't very good if there are millions of rows.
Long description: Say I have two models in my Django app, City and Country. City has a foreign key field to Country:
class Country(models.Model):
name = models.CharField(max_length = 256)
class City(models.Model):
name = models.CharField(max_length = 256)
population = models.IntegerField()
country = models.ForeignKey(Country, related_name = 'cities')
This means that a Country instance has .cities
available. Let's say I now want to write a method for Country called highest_city_population
that returns the population of the largest city. Coming from a LINQ background, my natural instinct is to try myCountry.cities.max('population')
or something like that, but this isn't possible.
Use Aggregation (new in Django 1.1). You use it like this:
>>> from django.db.models import Max
>>> City.objects.all().aggregate(Max('population'))
{'population__max': 28025000}
To get the highest population of a City
for each Country
, I think you could do something like this:
>>> from django.db.models import Max
>>> Country.objects.annotate(highest_city_population = Max('city__population'))
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