instance_cache.rb

module InstanceCache
  
  def self.included(receiver)
    receiver.extend(ClassMethods)
  end
  
  module ClassMethods
    # Override me for good times
    def find_by_cache_key(cache_key)
      find(cache_key)
    end
    
    def retrieve(cache_key)
      value = instance_cache[cache_key]
      if value.nil?
        value = find_by_cache_key(cache_key)
        set_cached_value(cache_key, value)
      elsif value == ExplicitNil.instance
        value = nil
      end
      value
    end
    
    def set_cached_value(cache_key, value)
      value = ExplicitNil.instance if value.nil?
      instance_cache[cache_key] = value
      value
    end
    
    def instance_cache
      @cache ||= {}
    end
    
    def preload_these(collection, &generate_cache_key)
      unless block_given?
        generate_cache_key = lambda{ |obj| obj.id }
      end
      
      collection.each do |obj|
        set_cached_value(generate_cache_key.call(obj), obj)
      end
    end
    
    def preload_all
      preload_these(all)
    end
    
    def instances(members)
      members.each do |name, value|
        instance_name = name.to_s.upcase
        const_name = "#{instance_name}_ID"
        self.const_set const_name, value
        self.class.send(:define_method, instance_name) do
          retrieve(value)
        end
      end
    end
  end
  
  class ExplicitNil < Object
    include Singleton
    def inspect
      "ExplicitNil"
    end
  end
end

class Example < ActiveRecord::Base
  include InstanceCache
  instances :foo => 1, :bar => 2
end

puts Example.FOO
puts Example::BAR_ID