设置关系属性将对象添加到设置关系的会话中有办法阻止这种情况吗？

问题描述

| 使用sqlalchemy，如果一个对象具有orm关系属性，并且您创建了该对象的实例并设置了该关系属性，则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

没有将bobs_addr添加到会话中，因此我希望它不会提交给数据库，但是设置bobs_addr.user = bob会导致将bobs_addr添加到会话中。如何阻止这种情况的发生？ 我知道有一个解决方法，那就是设置bobs_addr.user_id = bob.user_id，但是我更喜欢使用关系attr。     

解决方法

我认为这可以达到您想要的效果，但是我一直发现默认的

cascade=\'save-update,merge\'

效果最佳。

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