acts_as_cacheable.rb

module Acts
  module Cacheable
    def self.included(base)
      base.extend(ClassMethods)
    end

    module ClassMethods
      def acts_as_cacheable(key, options = {})
        extend Acts::Cacheable::SingletonMethods
        find(:all, options).each{|instance| cached_objects[instance.send(key).to_s] = instance}
      end
    end

    module SingletonMethods
      def [] (key)
        cached_objects[key]
      end

      def cached(id)
        cached_objects.each_value{|instance| return instance if instance.id == id}
      end

      def objects
        cached_objects.values
      end

   private
      def cached_objects
        @all_cached_objects ||= Hash.new
      end
   end
  end
end

person.rb

class Person < ActiveRecord::Base
  acts_as_cacheable :login, :conditions => {:admin => true}
end

unit.rb

class Unit < ActiveRecord::Base
  acts_as_cacheable :key
end