New and awesome gems

Paperclip: file uploads
- Images, audio, video, docs
SimpleForm, Nested Forms: form helpers
Thin, Puma, Unicorn: servers
- Thin: Rack-based, multiple servers to handle concurrency
- Puma: Multiple threads to handle concurrency, small memory footprint
- Unicorn: Handles load balancing and restarting workers on its own
- WEBrick: Basic server that is SLOW
- All can be used in conjuction with Apache/Phusion Passenger or nginx
Annotate: Route and Model enhancers
BetterErrors: exactly what it sounds like.
Pry: better debugging

Testing

What are we supposed to test?
TDD: test driven development
Red::Green::Refactor
How we test - Thoughtbot: http://robots.thoughtbot.com/how-we-test-rails-applications
Why We Don't Write Tests: https://whatdoitest.com/why-we-dont-write-tests
Writing Maintainable Tests: http://www.littlelines.com/blog/2013/12/17/a-guide-for-writing-maintainable-rails-tests/
We could spend an entire 12 weeks learning about testing - it's another DSL

Testing frameworks

Unit test
Mini test
RSpec
FactoryGirl
Capybara
Cucumber

The least you should know:

How to write a test
What is a good test vs what is an inconclusive test
How writing tests help you see the big picture while designing small systems in that scope