module ActiveRecord::Aggregations::ClassMethods

def reader_method(name, class_name, mapping, allow_nil, constructor)
  module_eval do
    define_method(name) do |*args|
      force_reload = args.first || false
      if (instance_variable_get("@#{name}").nil? || force_reload) && (!allow_nil || mapping.any? {|pair| !read_attribute(pair.first).nil? })
        attrs = mapping.collect {|pair| read_attribute(pair.first)}
        object = case constructor
          when Symbol
            class_name.constantize.send(constructor, *attrs)
          when Proc, Method
            constructor.call(*attrs)
          else
            raise ArgumentError, 'Constructor must be a symbol denoting the constructor method to call or a Proc to be invoked.'
          end
        instance_variable_set("@#{name}", object)
      end
      instance_variable_get("@#{name}")
    end
  end
end