The road to Rails 5

This post details the strategy and technical implementation of upgrading GitLab from Rails 4 to Rails 5. Key contributions include: developing a method to run the application on both Rails 4 and Rails 5 simultaneously using environment variables (`RAILS5`, `BUNDLE_GEMFILE`), splitting the upgrade into smaller, manageable issues, using a helper method (`Gitlab.rails5?`) to conditionally apply code for each Rails version, and a phased production release strategy to ensure stability. It also highlights challenges encountered with Active Record changes and monkey patches, and the importance of robust CI testing for both versions.