You really ought to consider rethinking why you need a global variable like this. Models are meant to stand along and be manipulated through an independent interface. So although you modify them from a controller, you can also use them in the console, in unit tests, or even completely separate ruby programs. Requiring a global variable instantly places a very specific constraint on all these possible uses.
That said, I see what you're trying to do, but I'm not sure of the best solution exactly. There must be a clean way to do it, because it's a cornerstone of 37signals apps. Maybe someone else can explain the best way to do it.
Michael Wilkes wrote: