cache_title.rb

# put this into yer initializers
#
# class Ad < ActiveRecord::Base
#   belongs_to :city
#   belongs_to :color
#   cache_title :city, :color
# end 
#
# then Ad.first.city_title

class ActiveRecord::Base
  def self.cache_title(*associations)

    self.class_eval do
      associations.each do |association|
        #TODO is there a better way to do this?
        association_class = self.new.send('build_'+association.to_s).class

        association_cache_name = association_class.to_s + '_titles'

        
        unless association_class.respond_to? 'cached_titles'
          (class << association_class; self; end).send(:define_method, :cached_titles) do
            Rails.cache.fetch(association_cache_name) do
              titles = {}
              association_class.all.each do |object|
                titles[object.id] = object.title
              end
              titles
            end
          end
        end

        define_method (association.to_s + '_title') do
          association_class.cached_titles[self.send(association.to_s + '_id')]
        end
      end
    end
  end
end