Upgrading Rails: Spelunking the Rails Code Base to Fix ActionMailer Test Failures

Recently, at work, I’ve had the pleasure of working on upgrading a legacy Ruby on Rails application from 4.2.11.1 to 5.2, and eventually v6.0. The app has been around a long time and has gone through many upgrades since the initial commit 10 years ago. Needless to say, most of the people who worked on the product in the past have moved on and aren’t available to provide context on decisions they made. Thankfully, the app is fairly well covered by unit and integration tests which have been tremendously helpful in surfacing problem areas.