带有OuterRef的简单子查询

Question

我正在尝试创建一个使用OuterRef的非常简单的子查询(不是为了实际目的,只是为了让它工作),但仍然遇到同样的错误.

文章/ models.py

from django.db import models

class Tag(models.Model):
    name = models.CharField(max_length=120)
    def __str__(self):
        return self.name

class Post(models.Model):
    title = models.CharField(max_length=120)
    tags = models.ManyToManyField(Tag)
    def __str__(self):
        return self.title

manage.py shell代码

>>> from django.db.models import OuterRef, Subquery
>>> from posts.models import Tag, Post
>>> tag1 = Tag.objects.create(name='tag1')
>>> post1 = Post.objects.create(title='post1')
>>> post1.tags.add(tag1)
>>> Tag.objects.filter(post=post1.pk)
<QuerySet [<Tag: tag1>]>
>>> tags_list = Tag.objects.filter(post=OuterRef('pk'))
>>> Post.objects.annotate(count=Subquery(tags_list.count()))

最后两行应该为每个Post对象提供标签数量.在这里我一直得到同样的错误:

ValueError: This queryset contains a reference to an outer query and may only be used in a subquery.

Answer 1

您的示例的一个问题是您不能queryset.count()用作子查询,因为.count()尝试评估查询集并返回计数.

所以人们可能会认为正确的方法是使用Count().也许是这样的:

Post.objects.annotate(
    count=Count(Tag.objects.filter(post=OuterRef('pk')))
)

这不会有两个原因:

1. 查询集Tag选择所有Tag字段,而Count只能依赖一个字段.因此:Tag.objects.filter(post=OuterRef('pk')).only('pk')需要(选择计数tag.pk).

2. Count本身不是一个Subquery阶级,Count是一个Aggregate.因此生成的表达式Count不被识别为a Subquery,我们可以通过使用来解决这个问题OuterRef.

对1)和2)的应用修复将产生:

Post.objects.annotate(
    count=Count(Subquery(Tag.objects.filter(post=OuterRef('pk')).only('pk')))
)

但是, 如果您检查正在生成的查询

SELECT 
    "tests_post"."id",
    "tests_post"."title",
    COUNT((SELECT U0."id" 
            FROM "tests_tag" U0 
            INNER JOIN "tests_post_tags" U1 ON (U0."id" = U1."tag_id") 
            WHERE U1."post_id" = ("tests_post"."id"))
    ) AS "count" 
FROM "tests_post" 
GROUP BY 
    "tests_post"."id",
    "tests_post"."title"

您可能会注意到我们有一个Subquery条款.这是因为Count是一个Aggregate,现在它不会影响结果,但在其他一些情况下它可能会影响结果.这就是为什么文档提出了一些不同的方法,其中聚合被移动到GROUP BY通过subquery+ values+ 的特定组合annotate

Post.objects.annotate(
    count=Subquery(
        Tag.objects.filter(post=OuterRef('pk'))
            # The first .values call defines our GROUP BY clause
            # Its important to have a filtration on every field defined here
            # Otherwise you will have more than one group per row!!!
            # This will lead to subqueries to return more than one row!
            # But they are not allowed to do that!
            # In our example we group only by post
            # and we filter by post via OuterRef
            .values('post')
            # Here we say: count how many rows we have per group 
            .annotate(count=Count('pk'))
            # Here we say: return only the count
            .values('count')
    )
)

最后这会产生:

SELECT 
    "tests_post"."id",
    "tests_post"."title",
    (SELECT COUNT(U0."id") AS "count" 
            FROM "tests_tag" U0 
            INNER JOIN "tests_post_tags" U1 ON (U0."id" = U1."tag_id") 
            WHERE U1."post_id" = ("tests_post"."id") 
            GROUP BY U1."post_id"
    ) AS "count" 
FROM "tests_post"

Answer 2

django -sql-utils包使这种子查询聚合变得简单。就pip install django-sql-utils在那时：

from sql_util.utils import SubqueryCount
posts = Post.objects.annotate(
            tag_count=SubqueryCount('tag'))

SubqueryCount 的 API 与 Count 相同，但它在 SQL 中生成子查询，而不是连接到相关表。