MongoEngine 模式 - 名称错误

MongoEngine schemata - name error

我在名为 model.py

的文件中有以下架构
from mongoengine import *

class Subject(Document):
    uri = StringField(required=True)
    resources = ListField(ReferenceField(ResourceSubject))

class ResourceSubject(Document):
    subject = ReferenceField(Subject,reverse_delete_rule=CASCADE)
    resource = ReferenceField(Resource)

class Resource(EmbeddedDocument):
    uri = StringField()
    title = StringField()
    snippet = StringField()
    image = StringField()
    source = StringField()
    adapter = StringField()

出于某种原因,当我尝试初始化一个主题时,subj = Subject(uri="hello").save() 我收到一个名称错误:NameError: name 'ResourceSubject' is not defined

我不明白为什么,我想这与框架有关?我尝试将 类 分隔在各个文件中,然后导入,但我仍然遇到相同的错误。我错过了什么?

这一行抛出错误:resources = ListField(ReferenceField(ResourceSubject))

ResourceSubject放入引号:

resources = ListField(ReferenceField('ResourceSubject'))