module CachedAttributes
  def self.included(klass)
    klass.extend ClassMethods
  end

  module ClassMethods
    def has_cached_attribute(name, default_value)
      # Only include the InstanceMethods module once

      after_initialize do |instance|
        store = instance.read_attribute(:cached_attrs) || {}

        val = default_value.respond_to?(:call) ? default_value.call(instance) : default_value
        store[name] = val

        instance.write_attribute(:cached_attrs, store) unless instance.attributes.keys.include?(name)
      end
    end
  end

  module InstanceMethods
    def cached_attr(name)
      (read_attribute(:cached_attrs) || {})[name]
    end
  end
end

ActiveRecord::Base.send(:include, CachedAttributes)