注释过滤 - 仅对一些相关对象的字段求和

Question

让我们说有一个作者,他有书.为了将作者与书面页面的数量一起获取,可以完成以下操作:

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

但是,如果我想分别对科幻和奇幻书籍的页面进行总结呢？我想最终得到一个具有total_pages_books_scifi_pages和total_pages_books_fantasy_pages属性的作者.

我知道我可以做以下事情:

Author.objects.filter(book__category='scifi').annotate(total_pages_books_scifi_pages=Sum('book__pages'))
Author.objects.filter(book__category='fantasy').annotate(total_pages_books_fantasy_pages=Sum('book__pages'))

但是如何在一个查询集中呢？

Answer 1

from django.db.models import IntegerField, F, Case, When, Sum

categories = ['scifi', 'fantasy']
annotations = {}

for category in categories:
    annotation_name = 'total_pages_books_{}'.format(category)
    case = Case(
        When(book__category=category, then=F('book__pages')),
        default=0,
        output_field=IntegerField()
    )
    annotations[annotation_name] = Sum(case)

Author.objects.filter(
    book__category__in=categories
).annotate(
    **annotations
)