sqlalchemy在web.py中的session使用

按照sqlalchemy的文檔中關於sqlalchemy的session在web應用內容相關的生命週期應該是：

Web Server          Web Framework        User-defined Controller Call
--------------      --------------       ------------------------------
web request    ->
                    call controller ->   # call Session().  this establishes a new,
                                         # contextual Session.
                                         session = Session()

                                         # load some objects, save some changes
                                         objects = session.query(MyClass).all()

                                         # some other code calls Session, it's the
                                         # same contextual session as "sess"
                                         session2 = Session()
                                         session2.add(foo)
                                         session2.commit()

                                         # generate content to be returned
return generate_content()
                    Session.remove() <-
web response   <-

見：http://www.sqlalchemy.org/docs/05/session.html#lifespan-of-a-contextual-session

不過web.py的cookbook中關於使用sqlalchemy的樣本中，最後並沒有釋放sqlalchemy的session資源，正確的應該如下：

def load_sqla(handler):
    web.ctx.orm = scoped_session(sessionmaker(bind=engine))
try:
return handler()
except web.HTTPError:
       web.ctx.orm.commit()
raise
except:
        web.ctx.orm.rollback()
raise
finally:
        web.ctx.orm.commit()
# If the above alone doesn't work, uncomment 
        # the following line:
        #web.ctx.orm.expunge_all() 
        web.ctx.orm.close() # <=- 關閉session，或者用 .remove()