reader_method

# File activerecord/lib/active_record/aggregations.rb, line 245
          def reader_method(name, class_name, mapping, allow_nil, constructor)
            define_method(name) do
              if @aggregation_cache[name].nil? && (!allow_nil || mapping.any? { |key, _| !read_attribute(key).nil? })
                attrs = mapping.collect { |key, _| read_attribute(key) }
                object = constructor.respond_to?(:call) ?
                  constructor.call(*attrs) :
                  class_name.constantize.send(constructor, *attrs)
                @aggregation_cache[name] = object
              end
              @aggregation_cache[name]
            end
          end