How can I retrieve a session based on the session-id

Rails
1

I am trying to mix Flash and RoR (for fileupload without pagerefresh)

Unfortunately the requests originating from the flashplayer do not have
the
right sessionid. So I tried to hackaround with reading the _session_id
form
the cookie and sending it together with the fileupload POST from the
flashplayer to the webserver.

But how can i retrieve the session based on the session_id at my
controller
?

I tried to add this to my action:

::ActionController::CgiRequest::DEFAULT_SESSION_OPTIONS.update(:session_id
=> params[:session_id])

and I hoped that the session for the request gets created based on
params[:session_id], but this is not the case.

So how do I retrieve from within an action a session from the
session-storage based on a sesison_id ?