[patch] Named scope can accept other named scopes as an argument

Hey all,

I've created a patch for Rails 2-3-stable that will allow named_scopes to use other defined named_scopes as arguments. This allows chaining named scopes within another named_scope.

For example:

class Post < ActiveRecord::Base
  named_scope :title_starts_with, lambda {|letter| {:conditions => ["title LIKE ?", letter + "%"]}}
  named_scope :title_starts_with_w, title_starts_with("W")
  named_scope :title_starts_with_w_limit_one, title_starts_with("W").limit(1)
end

What is this good for? It replicates the new arel-based query_interface, and could allow users using rails 2.3.9 (if included) to begin migrating their ActiveRecord queries early on, before Rails 3 is released into the wild.

An example of the power of this change -- I created a small gem called fake_arel that takes advantage of calling named scopes within other named scopes:
http://github.com/gammons/fake_arel

This gem implements all of the new rails 3 query interface finders using named scopes.

  Reply.where(:id => 1)
  Reply.select("content,id").where("id > 1").order("id desc").limit(1)
  Topic.joins(:replies).limit(1)

Attached is a patch for 2-3-stable. I look forward to your opinions.