Sluggo.

controller.rb

module Sluggo::Controller
  MATCHER = /(^.+)-(.*$)/
  
  def self.included(base)
    base.send :include, InstanceMethods
  end
  
  module InstanceMethods
    private
    
    def deslugify
      params[:id] = $2 if MATCHER =~ params[:id]
    end
  end
end

model.rb

module Sluggo::Model
  
  def self.included(base)
    base.send :extend,  ClassMethods
    base.send :include, InstanceMethods
    
    base.send :before_validation, :slugify
  end
  
  module ClassMethods
    attr_reader :slug_source_field, :slug_destination_field
    
    def sluggo(options={})
      @slug_source_field      = options[:source] || (raise ArgumentError, "sluggo requires a 'source'.")
      @slug_destination_field = options[:destination] || :slug
    end
  end
  
  module InstanceMethods
    def to_param
      send self.class.slug_destination_field
    end
    
    private
    
    def slugify
      prefix = send(self.class.slug_source_field).parameterize
      send "#{self.class.slug_destination_field}=", "#{prefix}-#{id}"
    end
  end
end