But I can't find anything even remotely similar to that in the API...
That's because, for the most part, you won't ever need to do that. The Rails architecture protects you from having to figure out that crap. If you need to, for example, delete the current session, you just do session.destroy. If you need to access a session variable for the current session, you just do session[:variable_of_interest]. As you learn more about Rails, you'll understand why that's true. If you have some reason for doing more than that kind of stuff, you'll need to say more about what it is you're trying to do. Most often, though, folks new to Rails find things like this are just much easier than they're used to.
I have three uses for grabbing a session by ID right now, and I'm betting as
I go along there will be more.
1) AJAX instant messanging. When two people initiate an IM conversation, I
would like the messages to be delivered by session ID.
2) PayPal. I would like the session that initiated a purchase to be
indicated in the PayPal IPN. I can do that without actually looking up the
session, but it would be nice to do something with the session (eg; set a
flag) when the IPN comes in. Yes, I could set a flag on the "user", but what
if somebody is making a purchase without creating an account?
3) Applets. These can't/don't always pass the browser's cookies along. It'd
be nice to point applets at a url with the session id embedded, and then
work with that to figure out what user/session is using the applet.
So all I really need, is some way to get from a Session ID to a session
object.
I will be, but I would still like a way to get from session ID to session
object without depending on active_record_store... that way I can turn this
stuff into plugins, without them depending on a model.