Proviso: I'm not sure whether this patch properly belongs in ActiveRecord or in ActionPack, since the bulk of the changes are in AR and the effects are most noticeable in AP...
This patch allows you to specify the field that should be returned from a model's to_param method (e.g., to_param(:title)). By default, you'll still get the id back like you always have - you'll just have the option of any other model field.
On its own, this isn't very useful. The patch also adds a little more magic to the routing code that takes advantage of the enhanced to_param. Say you have the following route defined:
map.profile 'members/:username', :controller => 'member', :action => 'profile'
Currently, you use that route with profile_url(:username => @member.username) or profile_url(@member.username) %>. With this patch, however, it's just profile_url(@member) - all the convenience you'd normally get if you were using the id field in the route, but with something much more meaningful to the end user.
The patch includes tests for the AR portion, and the AP tweak doesn't break any existing tests.
