如何使用 django orm 进行嵌套 Group By？

Question

我有以下数据：

publisher                    title
--------------------------  -----------------------------------
New Age Books                Life Without Fear
New Age Books                Life Without Fear
New Age Books                Sushi, Anyone?
Binnet & Hardley             Life Without Fear
Binnet & Hardley             The Gourmet Microwave
Binnet & Hardley             Silicon Valley
Algodata Infosystems         But Is It User Friendly?
Algodata Infosystems         But Is It User Friendly?
Algodata Infosystems         But Is It User Friendly?

这就是我想做的事情：我想计算单个对象中每个作者出版的书籍数量。 我想得到以下结果：

{publisher: New Age Books, titles: {Life Without Fear: 2, Sushi Anyone?: 1}},
{publisher: Binnet & Hardley, titles: {The Gourmet Microwave: 1, Silicon Valley: 1, Life Without Fear: 1}},
{publisher: Algodata Infosystems, titles: {But Is It User Friendly?: 3}} 

我的解决方案大致如下：

query_set.values('publisher', 'title').annotate(count=Count('title'))

但它没有产生预期的结果。

Answer 1

您可以使用itertools package [Python-doc] 的groupby(…) function [Python-doc] 对查询结果进行后处理：

from django.db.models import Count
from itertools import groupby
from operator import itemgetter

qs = query_set.values('publisher', 'title').annotate(
    count=Count('pk')
).order_by('publisher', 'title')

result = [
    {
        'publisher': p,
        'titles': {r['title']: r['count'] for r in rs }
    }
    for p, rs in groupby(qs, itemgetter('publisher'))
]