This plugin for ActiveRecord makes the "ID" field into a URL-safe GUID
It is a mashup by Andy Singleton <andy@assembla.com> that includes 
* the UUID class from Bob Aman.
* the plugin skeleton from Demetrius Nunes
* the 22 character URL-safe format from Andy Singleton
You can get standard 36 char UUID formats instead
TODO: Auto-detect a character ID field and use a GUID in this case (DRY principle)

It has been extended by Brian Morearty with:
* the addition of a mysql_create function (configurable with a guid_generator accessor)
  for much better performance
* id assignment is now done before_create instead of after_initialize, to more closely
  mimic the default Rails behavior of assigning an id upon save.

This library is free software; you can redistribute it and/or modify it
under the terms of the MIT license.

TO USE
Install as a plugin in the rails directory vendor/plugin/guid
define ID as char(22)
call "usesguid" in ActiveRecord class declaration, like
class Mymodel < ActiveRecord::Base
  usesguid      
end

if your ID field is not called "ID", call "usesguid :column =>'IdColumnName' "

If you use MySQL as your database, you can make guid generation much faster
by putting this in config/environment.rb:
  ActiveRecord::Base.guid_generator = :mysql