python - Python 中的 Google App Engine 循环模型依赖项

Question

我有以下 python 模块:

from google.appengine.ext import db
from user import User

class Post(db.Model):
    content = db.TextProperty()
    time = db.DateTimeProperty(auto_now_add=True)
    user = db.ReferenceProperty(User)
    topic = db.ReferenceProperty(Topic)

class Forum(db.Model):
    parent_ = db.SelfReferenceProperty()
    name = db.StringProperty()
    description = db.StringProperty()
    topics_count = db.IntegerProperty(default=0)
    posts_count = db.IntegerProperty(default=0)
    last_post = db.ReferenceProperty(Post)


class Topic(db.Model):
    forum = db.ReferenceProperty(Forum)
    title = db.StringProperty()
    replies_count = db.IntegerProperty(default=0)
    views = db.IntegerProperty(default=0)
    time = db.DateTimeProperty(auto_now_add=True)
    user = db.ReferenceProperty(User)
    last_post = db.ReferenceProperty(Post)

这不起作用，因为我在 Post 类中使用了 Topic 。我读过this solution并解决了循环依赖问题。生成的模块如下:

from google.appengine.ext import db
from user import User

class Post(db.Model):
    content = db.TextProperty()
    time = db.DateTimeProperty(auto_now_add=True)
    user = db.ReferenceProperty(User)


class Forum(db.Model):
    parent_ = db.SelfReferenceProperty()
    name = db.StringProperty()
    description = db.StringProperty()
    topics_count = db.IntegerProperty(default=0)
    posts_count = db.IntegerProperty(default=0)
    last_post = db.ReferenceProperty(Post)


class Topic(db.Model):
    forum = db.ReferenceProperty(Forum)
    title = db.StringProperty()
    replies_count = db.IntegerProperty(default=0)
    views = db.IntegerProperty(default=0)
    time = db.DateTimeProperty(auto_now_add=True)
    user = db.ReferenceProperty(User)
    last_post = db.ReferenceProperty(Post)


Post.topic = db.ReferenceProperty(Topic)

但是现在我有另一个问题。 App Engine 创建 Post 表，但它没有主题列。我怎么解决这个问题？

Answer 1

这种方法将无法正常工作，因为db.Model有一个元类，它在创建类时对数据库属性列表进行初始化工作。由于当时您的属性不存在，因此不会对其应用必要的初始化。

您可能需要考虑切换到 NDB，它通过使用关键属性之一的字符串名称来支持循环引用。