将post_save信号与抽象类

Question

我尝试使用创建和修改日期创建BaseDocument：

from datetime import datetime
from mongoengine import DateTimeField, Document, StringField, signals


class BaseDocument(Document):
    created_at = DateTimeField(default=datetime.utcnow)
    modified_at = DateTimeField()

    @classmethod
    def post_save(cls, sender, document, **kwargs):
        document.modified_at = datetime.utcnow()

    meta = {
        'abstract': True,
        'allow_inheritance': True
    }


signals.post_save.connect(BaseDocument.post_save, sender=BaseDocument)


class Book(BaseDocument):
    id = StringField(primary_key=True)

当我创建新的图书文档时：

>>> b = Book()
>>> b.id = '123'
>>> b.save()
<Book: Book object>

Mongo上未显示modified_at字段：

MongoDB shell version v3.4.0-rc2
connecting to: mongodb://127.0.0.1:27017
MongoDB server version: 3.4.0-rc2
> use test
switched to db test
> db.book.find()
{ "_id" : "123", "_cls" : "Book", "created_at" : ISODate("2016-11-08T18:17:20.761Z") }

我的方法有误吗？