Python的django OuterRef Exists Subquery

内容如下:

Use OuterRef when a queryset in a Subquery needs to refer to a field
from the outer query.

由上述文字可知，OuterRef是用于子查询语句的输出查询。
举个例子，假设由两个模型类如下：

class A(models.Model):
    id = models.CharField(max_length=36, default=uuid1, primary_key=True, editable=False)
    name = models.CharField(max_length=30, verbose_name='名称')
    code = models.CharField(max_length=2,  verbose_name='编码')
class B(models.Model):
    id = models.CharField(max_length=36, default=uuid1, primary_key=True, editable=False)
    name = models.CharField(max_length=30, verbose_name='名称')
    sub_id = models.CharField(max_length=36, verbose_name='父级id')

B.sub_id 存放的是A.id，即sub_id是A的外键字段

from django.db.models import OuterRef, Subquery，Exists
newest =A.objects.filter(id=OuterRef('sub_id'))
B.objects.annotate(newest=Exists(newest)).filter(newest=Ture)

则可查出B的关于存在A.id=B.sub_id的所有数据,当然你可以使用子查询,添加字段，这里的Exists本身就是一种子查询。

B.objects.annotate(newest_name=Subquery(newest.values('name')))