writer_method

# File activerecord/lib/active_record/aggregations.rb, line 164
        def writer_method(name, class_name, mapping, allow_nil, conversion)
          module_eval do
            define_method("#{name}=") do |part|
              if part.nil? && allow_nil
                mapping.each { |pair| @attributes[pair.first] = nil }
                instance_variable_set("@#{name}", nil)
              else
                part = conversion.call(part) unless part.is_a?(class_name.constantize) || conversion.nil?
                mapping.each { |pair| @attributes[pair.first] = part.send(pair.last) }
                instance_variable_set("@#{name}", part.freeze)
              end
            end
          end
        end