如何查询满足条件的相关记录的最大计数

Question

我正在开发一个Django应用，遇到了一些复杂的聚合查询，我想对其进行评估。

在我项目的demo应用中，我定义了几个模型类，用来表示图书馆的藏书情况：

from django.db import models

class Book(models.Model):
    interesting = models.BooleanField()

class Library(models.Model):
    books = models.ManyToManyField(Book)

我想查询的是单个图书馆中“有趣”书籍的最大数量，也就是每个图书馆中“有趣”书籍的数量的最大值。

在SQL中，这个查询可以写成：

select max(a.num_interesting_books) as max
from (select count(demo_book.id) as num_interesting_books
      from demo_book
      inner join demo_library_books on (demo_book.id = demo_library_books.book_id)
      where demo_book.interesting=TRUE
      group by demo_library_books.library_id) as a

 

使用以下测试数据：

insert into demo_library(id) values (1), (2), (3);
insert into demo_book(id, interesting) values
(1, FALSE), (2, FALSE), (3, TRUE),
(4, TRUE), (5, TRUE),
(6, TRUE), (7, TRUE), (8, TRUE), (9, FALSE);
insert into demo_library_books(library_id, book_id) values
(1, 1), (1, 2), (1, 3),
(2, 4), (2, 5),
(3, 6), (3, 7), (3, 8), (3, 9), (3, 3);

上面的SELECT语句的结果是：

 max
-----
   4
(1 row)

这正是我预期的结果。

请问是否可以使用Django的查询API来计算这个值呢？

Answer 1

我想我明白了：

Library.objects.filter(books__interesting=True).annotate(num_interesting_books=Count('pk')).aggregate(max=Max('num_interesting_books'))

翻译成SQL语言，这就是：

select max(a.num_interesting_books) as max
from (select demo_library.*, count(demo_library.id) as num_interesting_books
      from demo_library
      inner join demo_library_books on (demo_library.id = demo_library_books.library_id)
      inner join demo_book on (demo_library_books.book_id = demo_book.id)
      where demo_book.interesting=TRUE
      group by demo_library.id) as a