On Tue, Jan 8, 2013 at 2:16 PM, Michael Bayer <mike...@zzzcomputing.com>wrote:
> > On Jan 8, 2013, at 4:00 PM, Ken Lareau wrote: > > > Given this and your previous comments, and after some conversations with > a coworker, I decided to expose the 'engine' variable from the database > library and create a context manager as so: > > @contextlib.contextmanager > def isolated_transaction(NewSession=None): > """Manage a new transaction (session) within an existing session""" > > needed_session = True > existing = Session.registry() > > if NewSession is None: > NewSession = scoped_session(sessionmaker(bind=engine)) > else: > needed_session = False > > Session.registry.set(NewSession) > > try: > yield > finally: > Session.registry.set(existing) > > if needed_session: > NewSession.close() > > > (This code hasn't quite been tested yet and may need some tweaking) > With this, whenever I need an 'isolated' transaction I can simply do > from my program: > > with isolated_transaction(): > <do stuff with Session> > > And things should 'just work'... at least that's my hope! I can also pass > an existing new session if I want to use one in multiple places (not > sure I'll need that just yet, but it was easy enough to write into the > method, so...) > > Hopefully this seems like a sane solution to my problem. :) > > > its good, though you can save yourself all the trouble with > scoped_session/sessionmaker by just saying this: > > from sqlalchemy.orm import Session > new_session = Session(bind=engine) > > or which might be even more portable, use the sessionmaker from your > existing registry: > > new_session = Session.session_factory() > > session_factory is the sessionmaker() you stuck onto scoped_session in the > first place. In that case you don't need to refer to the Engine explicitly. > Nice, the second one definitely works and I no longer need to export 'engine' and only need to import Session from my library where I have the context manager place. :) Thanks again! - Ken >>> Depending on the database in use, using low isolation levels can have >>> the effect that other transactions can view "dirty reads" as the >>> transaction proceeds, but this is obviously an all-or-nothing thing. >>> When I need certain resources exposed during a long running transaction, I >>> transfer that data to a different Session and commit() those changes >>> distinctly. >>> >>> My current needs would tend to use the short transactions for things >> that are mostly >> isolated from anything going on in the longer running (main) transaction, >> though I do >> suspect I might need what you mention in your last sentence, but might >> you be able >> to refer me to an example of how it would work, perchance? >> >> >> I'd advise against going this route, you'd pretty much need to use MySQL >> MyISAM tables to get guaranteed "dirty reads", that is, there's no >> transaction at all, and it's not really how transactions were meant to be >> used. Lowering the isolation level is usually just a means to get more >> transaction throughput. >> > > Okay, I suspect I misunderstood what was being mentioned here, which is > no problem; I'll avoid it. :) Thanks once again for all the help. > > -Ken > > >> >> -- >> You received this message because you are subscribed to the Google Groups >> "sqlalchemy" group. >> To post to this group, send email to sqlalchemy@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. >> > > > > -- > - Ken Lareau > > > -- > You received this message because you are subscribed to the Google Groups > "sqlalchemy" group. > To post to this group, send email to sqlalchemy@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 sqlalchemy@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. > -- - Ken Lareau -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to sqlalchemy@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.
