instance method reader_method

# File activerecord/lib/active_record/aggregations.rb, line 214
        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