General
https://www.railstutorial.org
Robert C. Martin articles
Some conference videos: or not only for ruby ex (Boiling Frogs confs channel): https://www.youtube.com/channel/UCgUfIjfLvWmARsQ-d5gPzrw 
https://restfulapi.net/resource-naming/
http://apionrails.icalialabs.com/book/frontmatter

Clean code concepts and general advice
https://github.com/uohzxela/clean-code-ruby
http://rwdtow.stdout.in

Testing
https://thoughtbot.com/blog/how-we-test-rails-applications
https://dzone.com/articles/rspec-rails-controller-test
differences between Unit Tests and Integration Tests.
RSpec
Software Architecture meetings 
https://www.youtube.com/playlist?list=PLaBc6JOzoFjJ1lojIEDQh0nflUNUMzEQ7
Our very own tutorial from Tobiasz Waszak (in Polish)
https://www.youtube.com/watch?v=CXNCHUA3-4k