Question

我有以下（简化）模型：

class School(models.Model):
    name = models.CharField(max_length=128)

class UserProfile(models.Model):
    school = models.ForeignKey(School)

class Post(models.Model):
    profile = models.ForeignKey(UserProfile)

我想做的是获取一份学校清单，并注明每所学校的职位数量。出于多种原因，Post是外键键入UserProfile而不是School。我将如何使用Django ORM进行此操作？我猜我必须使用annotate（），但是当有问题的两个模型与外键没有直接关联时，它似乎不起作用。

Answer 1

使用lookups that span relationship与Generate aggregates for each item in a QuerySet：

from django.db.models import Count

School.objects.annotate(post_count=Count('userprofile__post'))

将使用这样的sql：

SELECT "testapp_school"."id", "testapp_school"."name", COUNT("testapp_post"."id") AS "post_count" FROM "testapp_school" LEFT OUTER JOIN "testapp_userprofile" ON ("testapp_school"."id" = "testapp_userprofile"."school_id") LEFT OUTER JOIN "testapp_post" ON ("testapp_userprofile"."id" = "testapp_post"."profile_id") GROUP BY "testapp_school"."id", "testapp_school"."name", "testapp_school"."id", "testapp_school"."name" LIMIT 21;

通过二级模型关系注释

1 个答案: