Let's continue from where I left off in Part 1 where I quickly described how you can extract reusable model logic from a Rails app into a testable Rubygem. If I were building a secondary Rails app connecting directly to the same database as the first, I could just add the dependency to the extracted gem: gem 'central-support', github: 'Codeminer42/cm42-central-support', branch: 'master', require: