On Jul 13, 2010, at 5:18 PM, Steven Wei wrote:
> I ran into an interesting behavior when working with SQLAlchemy 0.6.2
> and scoped_sessions: if you instantiate a Session before you call
> Session.configure(bind=engine), none of the Sessions you create will
> be properly bound to the engine you configured.
>
> Here's a short sample snippet that illustrates the issue:
>
> from sqlalchemy import create_engine, Column, Integer, String
> from sqlalchemy.ext.declarative import declarative_base
> from sqlalchemy.orm import scoped_session, sessionmaker
>
> engine = create_engine('sqlite:///:memory:',echo=True)
> Base = declarative_base()
>
> class User(Base):
> __tablename__ = 'users'
> id = Column(Integer, primary_key=True)
> name = Column(String)
>
> Base.metadata.create_all(engine)
>
> Session = scoped_session(sessionmaker())
> s1 = Session()
> Session.configure(bind=engine)
> s2 = Session()
>
> user = User(name='Bob')
>
> s2.add(user)
> s2.flush()
>
> Running this produces the following error:
>
> Traceback (most recent call last):
> File "sqlalchemy_bind.py", line 23, in <module>
> s2.flush()
> File "/opt/virtualenv/lib/python2.6/site-packages/sqlalchemy/orm/
> session.py", line 1346, in flush
> self._flush(objects)
> File "/opt/virtualenv/lib/python2.6/site-packages/sqlalchemy/orm/
> session.py", line 1427, in _flush
> flush_context.execute()
> File "/opt/virtualenv/lib/python2.6/site-packages/sqlalchemy/orm/
> unitofwork.py", line 299, in execute
> rec.execute(self)
> File "/opt/virtualenv/lib/python2.6/site-packages/sqlalchemy/orm/
> unitofwork.py", line 443, in execute
> uow
> File "/opt/virtualenv/lib/python2.6/site-packages/sqlalchemy/orm/
> mapper.py", line 1573, in _save_obj
> connection = uowtransaction.transaction.connection(self)
> File "/opt/virtualenv/lib/python2.6/site-packages/sqlalchemy/orm/
> session.py", line 246, in connection
> engine = self.session.get_bind(bindkey, **kwargs)
> File "/opt/virtualenv/lib/python2.6/site-packages/sqlalchemy/orm/
> session.py", line 868, in get_bind
> ', '.join(context)))
> sqlalchemy.exc.UnboundExecutionError: Could not locate a bind
> configured on mapper Mapper|User|users or this Session
>
> Presumably this is because first session (before the bind was
> configured) gets stored in the thread local registry, and gets
> returned on all future Session() instantiations.
>
> I've found two workarounds for this issue:
>
> 1) Don't instantiate Session objects until after you've called
> Session.configure(bind=engine). In some cases this might not be
> feasible, depending on the structure of your application.
> 2) Call Session.remove() after you call Session.configure(bind=engine)
> to clear the thread local registry, so you'll get a fresh instance of
> Session with the proper bind defined.
i wouldn't call those "workarounds", those are the correct steps to take, as
configure() is a proxy to the configure() of the underlying sessionmaker().
Docs seem to be a little light regarding configure(), the various docs are:
http://www.sqlalchemy.org/docs/reference/orm/sessions.html#sqlalchemy.orm.sessionmaker
http://www.sqlalchemy.org/docs/session.html?highlight=scoped_session#binding-session-to-an-engine.
http://www.sqlalchemy.org/docs/reference/orm/sessions.html#sqlalchemy.orm.scoping.ScopedSession.configure
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