Django 注释来自外部字典的字段值

Question

可以说我有以下命令：

schools_dict = {
   '1': {'points': 10},
   '2': {'points': 14},
   '3': {'points': 5},
}

我如何使用注释将这些值放入我的查询集中？ 我想这样做，但它不起作用

schools = SchoolsExam.objects.all()
queryset = schools.annotate(
    total_point = schools_dict[F('school__school_id')]['points']
)

楷模：

class SchoolsExam(Model):
    school = ForeignKey('School', on_delete=models.CASCADE),

class School(Model):
    school_id = CharField(),

这段代码给我一个错误KeyError: F(school__school_id)

Answer 1

您不能在查找中使用 F 对象，因为字典不“理解”F-对象。

您可以将其翻译成conditional expression ^[Django-doc]：

from django.db.models import Case, Value, When

schools = SchoolsExam.objects.annotate(
    total_point=Case(
        *[
            When(school__school_id=school_id, then=Value(v['points']))
            for school_id, v in school_dict.items()
        ]
    )
)

因此，这会将字典“展开”为CASE WHEN school_id=1 THEN 10 WHEN school_id=2 THEN 14 WHEN school_id=3 THEN 5。

然而，使用字典中的数据通常没有多大意义：通常将其存储在表中并执行 JOIN。