writer_method

# File activerecord/lib/active_record/aggregations.rb, line 235
        def writer_method(name, class_name, mapping, allow_nil, converter)
          define_method("#{name}=") do |part|
            if part.nil? && allow_nil
              mapping.each { |pair| self[pair.first] = nil }
              @aggregation_cache[name] = nil
            else
              unless part.is_a?(class_name.constantize) || converter.nil?
                part = converter.respond_to?(:call) ?
                  converter.call(part) :
                  class_name.constantize.send(converter, part)
              end

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