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Ruby on Ruby on Rails is designed under the MVC principles - Model View Controllers. Devise is a flexible authentication solution for Ruby on ROR projects. The solution is called devise and it is available in Ruby on Rack-Rack. The easiest way to use it is to create a Ruby-on-Racks app with the gem. The gem is a gem for anything you can think of and it can help you create a user-authentication solution for your application. For example, create a class called DeviseRegistrations and use it to authenticate users.