Rails Doctrine for general introduction.
Ruby on Rails Mailing List

Learn Rails by reading all the guides.
Learn SQL by Mode SQL tutorials.
The missing manual for Production Rails app.
Also checkout Upcase by Thoughtbot.

Note: Heroku sunset their free dynos, free Postgres, free Redis on November, 2022.

To grow your App into a Monolith. You can focus on building your ideas instead of all the other things. There is a entire talk about this and this thread:

Here is a real world case study.

On Writing Software well by DHH

It definitely scales with efforts and resources of course. Consult Speedshop Blog and Newsletter by Nate Berkopec for practical performance tips. Read engineering blogs from companies with scale.

Thanks core contributors like Aaron Patterson, Rafael Mendonça França, David Heinemeier Hansson, Jeremy Daer, Ryuta Kamizono, and other 5500+ people.

If you’re new and want to learn how to do things with Rails in real-life apps:

Active: bluedoc, rubyapi.org, dev.to, Lobste.rs, rubygems.org
Inactive but good to look at Pull Requests: Hound, Upcase

Top 10 errors from 1000+ Ruby on Rails projects (and how to avoid them)