设置关系attr将对象添加到设置关系的会话。有没有办法阻止它？

Question

对于sqlalchemy，如果有一个具有orm关系属性的对象，并且您创建了对象的实例并设置了关系属性，那么您的新对象将被添加到设置关系的会话中。例如：

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey 
from sqlalchemy.orm import relationship, Session 
from sqlalchemy.ext.declarative import declarative_base 

Base = declarative_base() 
class User(Base): 
    __tablename__ = 'users' 

    user_id = Column(Integer, primary_key=True) 
    name = Column(String) 

class Address(Base): 
    __tablename__ = 'addresses' 

    address_id = Column(Integer, primary_key=True) 
    user_id = Column(Integer, ForeignKey(User.user_id),) 
    address = Column(String) 

    user = relationship(User, backref='addresses') 


engine = create_engine('sqlite:///:memory:') 
Base.metadata.create_all(engine) 
session = Session(bind=engine) 

bob = User() 
bob.name = 'Bob' 
session.add(bob) 
session.commit() 

bobs_addr = Address() 
bobs_addr.user = bob 
bobs_addr.address = "23 Bob's st." 

# bobs_addr not added to the session 

engine.echo = True 
session.commit() 

运行上面的输出：

2011-05-23 12:07:23,461 INFO sqlalchemy.engine.base.Engine.0x...9a10 BEGIN (implicit) 
2011-05-23 12:07:23,463 INFO sqlalchemy.engine.base.Engine.0x...9a10 SELECT users.user_id AS users_user_id, users.name AS users_name 
FROM users 
WHERE users.user_id = ? 
2011-05-23 12:07:23,463 INFO sqlalchemy.engine.base.Engine.0x...9a10 (1,) 
2011-05-23 12:07:23,466 INFO sqlalchemy.engine.base.Engine.0x...9a10 INSERT INTO addresses (user_id, address) VALUES (?, ?) 
2011-05-23 12:07:23,466 INFO sqlalchemy.engine.base.Engine.0x...9a10 (1, "23 Bob's st.") 
2011-05-23 12:07:23,467 INFO sqlalchemy.engine.base.Engine.0x...9a10 COMMIT 

bobs_addr没有添加到会话，所以我希望它不会被提交到数据库，但设置bobs_addr.user = bob导致bobs_addr被添加到会话中。我如何阻止这种情况发生？

有一种解决方法，我知道，这是设置bobs_addr.user_id = bob.user_id，但我更愿意使用关系attr。

Answer 1

我认为这可以实现你想要的，但我总是发现默认cascade='save-update, merge'工作最好。

user = relationship(User, backref=backref('addresses', cascade=''))