DetachedInstanceError:Instance is not bound to a Session; attribute refresh operation cannot proced

有同事使用SQLAlchemy 遇到类似下边这样的报错：

DetachedInstanceError: Instance <User at 0x32768d0> is not bound to a Session; attribute refresh operation cannot proceed

搜索了下找到了答案，这种情况主要是发生在 关闭数据库会话后，使用ORM的对用，获取对象的某些字段值，这时就会报错。

报错的原因也很简单，SQLAlchemy的ORM方式将数据库中的记录映射成了我们定义好的模型类，这些类对象的实例只在数据库会话（session）的生命期内有效，假如我们将数据库会话关闭了，再访问数据表类的对象就会报错。类似代码如下：

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class User(Base):
    __tablename__ = 'USER'

    id = Column(Integer, Sequence('USER_SEQ'), primary_key=True, autoincrement=True)
    name = Column(String(30))
    age = Column(Integer)
    status = Column(String(30))


engine = create_engine(str(db_url), encoding=b'utf-8', echo=echo, convert_unicode=True)
Session = sessionmaker(bind=engine)
session = Session()

user = User(name='John', age=30)
session.add(user)
session.commit()
session.close()

print user.name

运行到最后一行会抛出异常：

Traceback (most recent call last):
  File "E:\WorkCopy\test\test.py", line 21, in test_something
    print user.name
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\attributes.py", line 237, in __get__
    return self.impl.get(instance_state(instance), dict_)
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\attributes.py", line 573, in get
    value = state._load_expired(state, passive)
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\state.py", line 480, in _load_expired
    self.manager.deferred_scalar_loader(self, toload)
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\loading.py", line 610, in load_scalar_attributes
    (state_str(state)))
DetachedInstanceError: Instance <User at 0x32768d0> is not bound to a Session; attribute refresh operation cannot proceed

 那么如何在关闭Session 后还可以正常使用这个对象呢？ 使用 session.expunge()。

user = User(name='John', age=30)
session.add(user)
session.commit()

session.refresh(user)
session.expunge(user)

session.close()

print user.name

使用 session.refresh(user) 是为了读取自增字段值（如果有的话）到 user 对象

 

问题参考链接：

https://segmentfault.com/q/1010000004530775