I personally found that the most useful diagram to help me understand a Rails project I don't know is the E/R automatically generated from this tool: https://gi

I personally found that the most useful diagram to help me understand a Rails project I don't know is the E/R automatically generated from this tool: https://github.com/voormedia/rails-erd https://github.com/voormedia/rails-erd Being automatically generated, it's always up to date. As many others noted, I never had luck with written docs because it never happened to me to find good ones which were in sync with the project.