On Mar 28, 2010, at 10:26 AM, Peteris Krumins wrote: > Hi all, > > Can anyone explain to me how to correctly create sessions in web > applications? > > The documentation gives an example with a scoped_session: > > from sqlalchemy.orm import scoped_session, sessionmaker > Session = scoped_session(sessionmaker()) > > Okay, I should wrap the "thing" returned by sessionmaker() in > scoped_session(). But do I later still have to construct Session() by > doing this: > > session = Session() > > I did some experiments and I don't get it. > > Here is the code I tried: > >>>> from app.models import Page >>>> >>>> Session = sessionmaker(bind=engine, autoflush=True, > autocommit=False) >>>> session = scoped_session(Session) >>>> >>>> session > <sqlalchemy.orm.scoping.ScopedSession object at 0x848eb8c> >>>> session() > <sqlalchemy.orm.session.Session object at 0x83fe08c> >>>> session.query(Page).count() > 211L >>>> session().query(Page).count() > 211L > > Both work identically, but I don't understand the difference. Can > anyone explain it please? > > Also when do I have to call remove() method that is mentioned in > documentation?
ScopedSession is a class that implements __call__(). An instance of ScopedSession then maintains a reference to a thread local object, which in turn references individual Session objects. So if "ss" is a ScopedSession instance, calling ss() (i.e. its __call__() method) returns the current thread's Session. In this way ScopedSession mimics a "constructor", and an existing application which says "sess = Session(); sess.query(Foo)" can transparently be made to use a thread-scoped session by changing the meaning of the identifier "Session". The second part of the story is that people found it tedious to say sess = Session(); sess.query(Foo) all the time to perform an operation on the current thread's session, so a __getattr__()-like scheme was added which compresses those two steps into one, meaning <some_scoped_session>.<some_Session_method>() equates to "sess = Session(); return sess.<some_Session_method>()". But what this also means is, like the case of __call__(), an application which currently uses a single global Session to do everything, i.e. "session.query(Foo)" throughout, can also transparently be made to use a thread-scoped session by changing the meaning of the identifier "session". > > > Thanks, > Peteris Krumins > > -- > You received this message because you are subscribed to the Google Groups > "sqlalchemy" group. > To post to this group, send email to sqlalch...@googlegroups.com. > To unsubscribe from this group, send email to > sqlalchemy+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/sqlalchemy?hl=en. > -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to sqlalch...@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
