gistfile1.rb

module SetNotationHelper
  def self.included(base)
    base.extend(ClassMethods)
  end
  
  module ClassMethods

    # Here's our little magic hook
    def inherited(subclass)  
      subclass.class_eval do
        named_scope :in_set, lambda{|*args|
          { :conditions => "#{table_name}.id IN (#{ parse_set(args.first).to_a.join(',') })" }
        }
      end
    end

  end
end

ActiveRecord::Base.send :include, SetNotationHelper