from django.db import modelsclass Author(models.Model):name = models.CharField(max_length=100)age = models.IntegerField()class Publisher(models.Model):name = models.CharField(max_length=300)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, on_delete=models.CASCADE)pubdate = models.DateField()class Store(models.Model):name = models.CharField(max_length=300)books = models.ManyToManyField(Book)

# Book的总数
>>> Book.objects.count()
2452# 符合publisher=BaloneyPress条件的Book总数
>>> Book.objects.filter(publisher__name='BaloneyPress').count()
73# 所有Book价格的平均值
>>> from django.db.models import Avg
>>> Book.objects.all().aggregate(Avg('price'))
{'price__avg': 34.35}# 所有Book中价格的最大值
>>> from django.db.models import Max
>>> Book.objects.all().aggregate(Max('price'))
{'price__max': Decimal('81.20')}# 所有Book中最高价与平均值的差值
>>> from django.db.models import FloatField
>>> Book.objects.aggregate(
...     price_diff=Max('price', output_field=FloatField()) - Avg('price'))
{'price_diff': 46.85}# 以下请求都包含了通过Book<->Publisher外键实现对反向关系的查询# 每个出版社出版书籍的数量，统计到num_books属性上
>>> from django.db.models import Count
>>> pubs = Publisher.objects.annotate(num_books=Count('book'))
>>> pubs
<QuerySet [<Publisher: BaloneyPress>, <Publisher: SalamiPress>, ...]>
>>> pubs[0].num_books
73# 针对书籍评价等级，统计每个出版商评级等级大于5和小于等于5的书籍数量是多少
>>> from django.db.models import Q
>>> above_5 = Count('book', filter=Q(book__rating__gt=5))
>>> below_5 = Count('book', filter=Q(book__rating__lte=5))
>>> pubs = Publisher.objects.annotate(below_5=below_5).annotate(above_5=above_5)
>>> pubs[0].above_5
23
>>> pubs[0].below_5
12# 出版书籍数量最多的五个出版商
>>> pubs = Publisher.objects.annotate(num_books=Count('book')).order_by('-num_books')[:5]
>>> pubs[0].num_books
1323

>>> Book.objects.all()

>>> from django.db.models import Avg
>>> Book.objects.all().aggregate(Avg('price'))
{'price__avg': 34.35}

>>> Book.objects.aggregate(Avg('price'))
{'price__avg': 34.35}

>>> Book.objects.aggregate(average_price=Avg('price'))
{'average_price': 34.35}

>>> from django.db.models import Avg, Max, Min
>>> Book.objects.aggregate(Avg('price'), Max('price'), Min('price'))
{'price__avg': 34.35, 'price__max': Decimal('81.20'), 'price__min': Decimal('12.99')}

# 构建一个annotate聚合的QuerySet
>>> from django.db.models import Count
>>> q = Book.objects.annotate(Count('authors'))
# 查看queryset中第一个对象
>>> q[0]
<Book: The Definitive Guide to Django>
>>> q[0].authors__count
2
# 查看queryset中第二个对象
>>> q[1]
<Book: Practical Django Projects>
>>> q[1].authors__count
1

>>> q = Book.objects.annotate(num_authors=Count('authors'))
>>> q[0].num_authors
2
>>> q[1].num_authors
1

>>> book = Book.objects.first()
>>> book.authors.count()
2
>>> book.store_set.count()
3
>>> q = Book.objects.annotate(Count('authors'), Count('store'))
>>> q[0].authors__count
6
>>> q[0].store__count
6

>>> q = Book.objects.annotate(Count('authors', distinct=True), Count('store', distinct=True))
>>> q[0].authors__count
2
>>> q[0].store__count
3

为了弄明白请求做了什么，可以通过QuerySet的query属性检验下。

>>> from django.db.models import Max, Min
>>> Store.objects.annotate(min_price=Min('books__price'), max_price=Max('books__price'))

>>> Store.objects.aggregate(min_price=Min('books__price'), max_price=Max('books__price'))

>>> from django.db.models import Avg, Count, Min, Sum
>>> Publisher.objects.annotate(Count('book'))

>>> Publisher.objects.aggregate(oldest_pubdate=Min('book__pubdate'))

>>> Author.objects.annotate(total_pages=Sum('book__pages'))

>>> Author.objects.aggregate(average_rating=Avg('book__rating'))

>>> from django.db.models import Avg, Count
>>> Book.objects.filter(name__startswith="Django").annotate(num_authors=Count('authors'))

>>> Book.objects.filter(name__startswith="Django").aggregate(Avg('price'))

>>> Book.objects.annotate(num_authors=Count('authors')).filter(num_authors__gt=1)

>>> highly_rated = Count('books', filter=Q(books__rating__gte=7))
>>> Author.objects.annotate(num_books=Count('books'), highly_rated_books=highly_rated)

选用filter参数，还是QuerySet.filter()在单独的annotate标注或aggregate聚合中，要避免使用filter参数的形式。对于排除某些行而言，使用QuerySet.filter()更有效率。聚合函数所用的filter参数仅在基于同一关系但是条件不同的两个及以上的聚合中才有用。

Django2.0以来的改变
聚合中增加了filter参数。

>>> a, b = Publisher.objects.annotate(num_books=Count('book', distinct=True)).filter(book__rating__gt=3.0)
>>> a, a.num_books
(<Publisher: A>, 2)
>>> b, b.num_books
(<Publisher: B>, 2)>>> a, b = Publisher.objects.filter(book__rating__gt=3.0).annotate(num_books=Count('book'))
>>> a, a.num_books
(<Publisher: A>, 2)
>>> b, b.num_books
(<Publisher: B>, 1)

>>> a, b = Publisher.objects.annotate(avg_rating=Avg('book__rating')).filter(book__rating__gt=3.0)
>>> a, a.avg_rating
(<Publisher: A>, 4.5)  # (5+4)/2
>>> b, b.avg_rating
(<Publisher: B>, 2.5)  # (1+4)/2>>> a, b = Publisher.objects.filter(book__rating__gt=3.0).annotate(avg_rating=Avg('book__rating'))
>>> a, a.avg_rating
(<Publisher: A>, 4.5)  # (5+4)/2
>>> b, b.avg_rating
(<Publisher: B>, 4.0)  # 4/1 (book with rating 1 excluded)

>>> Book.objects.annotate(num_authors=Count('authors')).order_by('num_authors')

>>> Author.objects.annotate(average_rating=Avg('book__rating'))

>>> Author.objects.values('name').annotate(average_rating=Avg('book__rating'))

>>> Author.objects.annotate(average_rating=Avg('book__rating')).values('name', 'average_rating')

from django.db import modelsclass Item(models.Model):name = models.CharField(max_length=10)data = models.IntegerField()class Meta:ordering = ["name"]

# 警告: 并不十分正确！
Item.objects.values("data").annotate(Count("id"))

Item.objects.values("data").annotate(Count("id")).order_by()

注意很自然，我们就会对django为什么不删除无关列产生疑问。主要原因是要与distinct()等保持一致：Django从不删除您指定的排序约束（并且我们也无法更改其他方法的行为，因为会违反API稳定性策略）。

>>> from django.db.models import Avg, Count
>>> Book.objects.annotate(num_authors=Count('authors')).aggregate(Avg('num_authors'))
{'num_authors__avg': 1.66}