Mongoengine中带有reverse_delete_rule的循环依赖答案

【问题标题】：Circular dependency with reverse_delete_rule in MongoengineMongoengine中带有reverse_delete_rule的循环依赖
【发布时间】：2017-06-30 11:27:22
【问题描述】：

我有以下相互依赖的模型：

from mongoengine import *

class DocumentA(Document):
    docB = ReferenceField('DocumentB', reverse_delete_rule=CASCADE)

class DocumentB(Document):
    docA = ReferenceField('DocumentA', reverse_delete_rule=CASCADE)

我收到以下错误：

mongoengine.errors.NotRegistered: `DocumentB` has not been registered in the document registry.
            Importing the document class automatically registers it, has it
            been imported?

如果我删除第一个reverse_delete_rule，这个错误就会消失：

from mongoengine import *

class DocumentA(Document):
    docB = ReferenceField('DocumentB')

class DocumentB(Document):
    docA = ReferenceField('DocumentA', reverse_delete_rule=CASCADE)

工作正常。

有没有办法保留reverse_delete_rule？

【问题讨论】：

标签： python mongoengine circular-dependency

【解决方案1】：

来自MongoEngine ReferenceField documentation：

注册删除规则的替代语法（在以下情况下很有用）实现双向删除规则）
class Bar(Document):
    content = StringField()
    foo = ReferenceField('Foo')

Foo.register_delete_rule(Bar, 'foo', NULLIFY)

所以你的代码变成了：

from mongoengine import *

class DocumentA(Document):
    docB = ReferenceField('DocumentB')

class DocumentB(Document):
    docA = ReferenceField('DocumentA', reverse_delete_rule=CASCADE)

DocumentB.register_delete_rule(DocumentA, 'docB', CASCADE)

【讨论】：