Just Enough Ruby

Experienced Rails developers debate whether beginners should study Ruby
before learning Rails.

By all means, if you love the precision and order of programming
languages, dive into the study of Ruby from the beginning. But most
people don’t delay starting Rails while learning Ruby; realistically,
you’ll retain more knowledge of Ruby if you learn it as you build things
in Rails. That is the approach we’ve taken in this book. You’ve already
built a simple Rails application and used Ruby as you did so.