我正在尝试在 sqlalchemy 和我的 postgres 数据库之间添加一个 pylibmc memcached 层。 This post谈到了如何去做。但我也想要反向引用(用于在关系中从一个到多个)，所以我修改了该帖子的 user_classes.py 来测试它。
我使 UserStatus 继承自 database.MemcachedORMObject 并在 UserTable.mapper 中添加了一个 backref。但在尝试访问反向引用时，我收到“DetachedInstanceError:父实例未绑定(bind)到 session ；属性“用户”的延迟加载操作无法继续”。我正在使用 Flask-SQLAlchemy==0.16、SQLAlchemy==0.8.0b2 和 pylibmc==1.2.3。
class UserStatus(database.MemcachedORMObject):
def __init__(self, name):
self.name = name
UserStatusTable = Table('user_status', METADATA, \
Column('user_status_id', Integer, primary_key=True),
Column('name', String) )
UserStatusTable.mapper = mapper(UserStatus, UserStatusTable)
UserStatusTable.mapper.compile()
class User(database.MemcachedORMObject):
def __init__(self, name, email, password, user_status_id):
self.name = name
self.email = email
self.password = password
self.user_status_id = user_status_id
UserTable = Table('user', METADATA, \
Column('user_id', Integer, primary_key=True),
Column('name', String),
Column('email', String),
Column('password', String),
Column('user_status_id', Integer),
ForeignKeyConstraint(['user_status_id'], ['user_status.user_status_id']) )
UserTable.mapper = mapper(User, UserTable,
properties = { 'user_status': relation(UserStatus, backref='user', lazy=False)})
UserTable.mapper.compile()
尝试访问反向引用时出错:
In [14]: from user_classes import * In [15]: ust = UserStatus.fetch_by_field(UserStatus.user_status_id, 1) Memcached Getting: user_classes.UserStatus:(1) In [16]: ust.user
--------------------------------------------------------------------------- DetachedInstanceError Traceback (most recent call last) <ipython-input-16-38fd625abed9> in <module>()
----> 1 ust.user /home/david/sqlalchemy-memcached/env/lib/python2.7/site-packages/sqlalchemy/orm/attribut es.pyc in __get__(self, instance, owner)
249 return dict_[self.key]
250 else:
--> 251 return self.impl.get(instance_state(instance), dict_)
252
253 /home/david/sqlalchemy-memcached/env/lib/python2.7/site-packages/sqlalchemy/orm/attribut es.pyc in get(self, state, dict_, passive)
543 value = callable_(passive)
544 elif self.callable_:
--> 545 value = self.callable_(state, passive)
546 else:
547 value = ATTR_EMPTY /home/david/sqlalchemy-memcached/env/lib/python2.7/site-packages/sqlalchemy/orm/strategi es.pyc in _load_for_state(self, state, passive) 495 "Parent instance %s is not bound to a Session; "
496 "lazy load operation of attribute '%s' cannot proceed" %--> 497 (orm_util.state_str(state), self.key)
498 )
499 DetachedInstanceError: Parent instance <UserStatus at 0x21e9dd0> is not bound to a Sessi on; lazy load operation of attribute 'user' cannot proceed
最佳答案
这是 SQLAlchemy 中的一个错误。我在错误跟踪器上报告了它:http://www.sqlalchemy.org/trac/ticket/2743
您始终可以遍历默认关系方向。因此，您的情况的解决方法是更改关系的方向(以便您可以从用户状态访问用户，但反之则不然)。
一般的解决方法(如果您稍后需要遍历两个方向)是修补定义 backref 的对象中的 __dict__ 属性。对于错误跟踪器上的测试用例，这将是:
for p in parents:
for c in p.children:
getattr(c, '__dict__')['parent'] = p
print parents[0].children[0].parent
关于python - 分离的 SQLalchemy session 无法延迟加载 backref 对象，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/15188444/