Django Aggregation聚合使用方法解析聚合使用方法解析
主要介绍了Django Aggregation聚合使用方法解析,文中通过示例代码介绍的非常详细，对大家的学习或者工作
具有一定的参考学习价值,需要的朋友可以参考下
在当今根据需求而不断调整而成的应用程序中，通常不仅需要能依常规的字段，如字母顺序或创建日期，来对项目进行排序，
还需要按其他某种动态数据对项目进行排序。Djngo聚合就能满足这些要求。
以下面的Model为例
from django.db import models
class Author(models.Model):
name = models.CharField(max_length=100)
age = models.IntegerField()
class Publisher(models.Model):
name = models.CharField(max_length=300)
num_awards = models.IntegerField()
class Book(models.Model):
name = models.CharField(max_length=300)
pages = models.IntegerField()
price = models.DecimalField(max_digits=10, decimal_places=2)
rating = models.FloatField()
authors = models.ManyToManyField(Author)
publisher = models.ForeignKey(Publisher)
pubdate = models.DateField()
class Store(models.Model):
name = models.CharField(max_length=300)
books = models.ManyToManyField(Book)
registered_users = models.PositiveIntegerField()
快速了解快速了解
# books总数量.
>>> Book.objects.count()
2452
# Total number of books with publisher=BaloneyPress
>>> Book.objects.filter(publisher__name='BaloneyPress').count()
73
# books的平均price.
>>> from django.db.models import Avg
>>> Book.objects.all().aggregate(Avg('price'))
{'price__avg': 34.35}
# books的最大price.
>>> from django.db.models import Max
>>> Book.objects.all().aggregate(Max('price'))
{'price__max': Decimal('81.20')}
# All the following queries involve traversing the Book<->Publisher
# many-to-many relationship backward
# 为每个publisher添加个num_books属性，即每个pulisher出版的book的数量.
>>> from django.db.models import Count
>>> pubs = Publisher.objects.annotate(num_books=Count('book'))
>>> pubs
[<Publisher BaloneyPress>, <Publisher SalamiPress>, ...]
>>> pubs[0].num_books
73
# 根据num_book属性排序.
>>> pubs = Publisher.objects.annotate(num_books=Count('book')).order_by('-num_books')[:5]
>>> pubs[0].num_books
1323
聚合生成聚合生成Generating aggregates over a QuerySet
Django有两种方法来生成聚合。第一种方法是为整个QuerySet生成聚合值，例如为全部的books生成price的平均值：
>>> from django.db.models import Avg
>>> Book.objects.all().aggregate(Avg('price'))
{'price__avg': 34.35}
可以简略为：
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