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

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)

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