A practical guide to upgrading from Rails 4.2 to Rails 5 with breaking changes, Ruby version requirements, and step-by-step migration strategies.

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Rails 5 requires Ruby 2.2.2+ and introduces breaking changes including mandatory belongs_to associations, callback chain modifications (throw :abort instead of false), migration versioning, and controller test parameter syntax updates. The upgrade process involves updating the Gemfile, running rails app:update, replacing deprecated methods (before_filter → before_action, render :text → render plain:), and addressing ActiveRecord relation improvements. Expected timeline is 2-4 weeks for medium-sized applications with 80%+ test coverage. Performance improvements include 10-15% faster request processing and better memory usage with Ruby 2.4.

Upgrading from Rails 4.2 to Rails 5 - A Complete Guide