使用 django orm 写 exists 条件过滤实例

要用django的orm表达sql的exists子查询，是个比较麻烦的事情，需要做两部来完成

from django.db.models import Exists, OuterRef

# 1. 定义子查询条件
relative_comments = Comment.objects.filter(
post=OuterRef('pk'), # 注意外键关联方式:post为Comment表的字段，pk表示关联另一表主键
)

# 2. 使用annotate和filter共同定义子查询
Post.objects.annotate( # 使用exists定义一个额外字段
recent_comment=Exists(recent_comments),
).filter(recent_comment=True) # 在条件中通过检查额外字段实现exists子查询过滤

这种方式比较麻烦，有其它简便方式的欢迎分享

官网参考： https://docs.djangoproject.com/en/2.1/ref/models/expressions/#filtering-on-a-subquery-expression

补充知识：关于使用django orm 时的坑

跨app 时外键报错

class Host(models.Model):
nid = models.AutoField(primary_key=True)
hostname = models.CharField(max_length=32, db_index=True)
ip = models.GenericIPAddressField(protocol=“ipv4”, db_index=True)
port = models.IntegerField()
# b = models.ForeignKey(to=“Business”, to_field=‘id')

class HostToApp(models.Model):
hobj = models.ForeignKey(to=‘Host', to_field=‘nid')
aobj = models.ForeignKey(to=‘Application', to_field=‘id')

class Application(models.Model):
name = models.CharField(max_length=32)

以上 model 都在一个models 文件下时不会报错。 但是一旦出现跨app 时会报以下错误：

users.HostToApp.aobj: (fields.E300) Field defines a relation with model ‘Application', which is either not installed, or is abstract.
users.HostToApp.aobj: (fields.E307) The field users.HostToApp.aobj was declared with a lazy reference to ‘users.application', but app ‘users' doesn't provide model ‘application'.

解决方案：

1、

from xxxx.models import Application

2、

class HostToApp(models.Model):
hobj = models.ForeignKey(to=‘Host', to_field=‘nid')
aobj = models.ForeignKey(to=‘xxxx.Application', to_field=‘id')

第二步很重要

来源：https://blog.csdn.net/xufwind/article/details/86476597