Rails Core developer Jose Valim guides you through seven different tutorials, each of them using test-driven development to build a new Rails extension or application that solves common problems with these new APIs. You will understand how the Rails rendering stack works and customize it to read templates from the database while you learn how to mimic Active Record behavior, like validations, in any other object. You will find out how to write faster, leaner controllers, and you'll learn how to mix Sinatra applications into your Rails apps, so you can choose the most appropriate tool for the job. In addition, you will improve your productivity by customizing generators and responders.