django - 注释过滤——只对一些相关对象的字段求和

Question

假设有一个 Author 并且他有 Books。为了获取作者和已写页数，可以执行以下操作:

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求和怎么办？我想最终得到一个作者，它具有 total_pages_books_scifi_pages 和 total_pages_books_fantasy_pages 属性。

我知道我可以执行以下操作:

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做到这一点？

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
)