Rebuilding Rails: What Does a Rails Expert Know About Debugging That You Don't?

This was written by the recently deceased Noah Gibbs. The site is now under the control of his widow. It's a great course and I'd encourage people to check it out.

Brilliant idea. In order to learn more about Ruby on Rails, the course takes you on a journey in which you build Ruby on Rails from scratch.

This could go a long way towards removing the magic and actually understanding how Rails works, and why it is designed that way.