In polymorphic_routes.rb, polymorphic_url is defined in rails source from github,and this particular line:

   inflection = if options[:action] && options[:action].to_s == "new"           args.pop           :singular         elsif (record.respond_to?(:persisted?) && !record.persisted?)           args.pop           :plural         elsif record.is_a?(Class)           args.pop           :plural         else           :singular         end

what's the purpose of extracting last element of array here? At this point args will either be an array of activerecord objects, or just be a single activerecord object in an array.

Oh, I figured this one out. Reason why the else doesn't pop the last element is because the last element is the object passed to form_for that has been persisted, and that element is needed in the array for the id value when we invoke send on the dynamic route generated.