ruby-debug19 is not maintained anymore. All the other answers are outdated.
But there's an alternative:
debugger to the rescue!
Put this in your Gemfile:
gem 'debugger', group: [:development, :test]
It just works. - And it is included in the rails Gemfile since 3.2.something to replace ruby-debug19. It has the exact same functionality and is actively maintained.
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