Darn-near everything I know about writing and maintaining a Rails app that stays a joy to work on. I also snuck a whole lot of generally-good OOP advice in there.
The course is 2.5 hours of video spread over 8 lessons. It also includes the source code shown in all videos.
Topics covered include form objects, page objects, responsible callback use, improving test speed, "Tell, Don't Ask", testing best practices, and more.
For a longer course description and FAQ: https://www.refactoringrails.io/buy