instance method writer_method

# File activerecord/lib/active_record/aggregations.rb, line 251
        def writer_method(name, class_name, mapping, allow_nil, converter)
          module_eval do
            define_method("#{name}=") do |part|
              if part.nil? && allow_nil
                mapping.each { |pair| self[pair.first] = nil }
                instance_variable_set("@#{name}", nil)
              else
                unless part.is_a?(class_name.constantize) || converter.nil?
                  part = case converter
                    when Symbol
                     class_name.constantize.send(converter, part)
                    when Proc, Method
                      converter.call(part)
                    else
                      raise ArgumentError, 'Converter must be a symbol denoting the converter method to call or a Proc to be invoked.'
                    end
                end

                mapping.each { |pair| self[pair.first] = part.send(pair.last) }
                instance_variable_set("@#{name}", part.freeze)
              end
            end
          end
        end