junwei

ok, let me separate the change from refactoring thank you for the advice. have a nice vacation :D

Updated: only include desired changes for performance optimization

hello @bf4 , sorry to bother , just would like to see whether the changes looks good to you ? Orz

> Make sure you have [Ragel ~> 6.7](http://www.complang.org/ragel/) installed

> [Rewriting](http://whitequark.org/blog/2013/04/26/lets-play-with-ruby-code/) support.

hi everyone, just would like to bring this up again as we spot performance issue in [lib/active_model/serializer/fieldset.rb](https://github.com/rails-api/active_model_serializers/blob/9fa9373f949f917a8ab22e7bfdfd5829c53d0bcb/lib/active_model/serializer/fieldset.rb#L15) as well our temp solution is patch `lib/active_model/serializer/fieldset.rb@fields_for`  and it saves about...