Simply use config.force_ssl = true in your environment configuration.

You can also selectively enable https depending on the current Rails environment. For example, you might want to keep HTTPS turned off on development, and enable it on staging/production.

# config/application.rb
module MyApp
  class Application < Rails::Application
    config.force_ssl = false
  end
end

# config/environments/production.rb
MyApp::Application.configure do
  config.force_ssl = true
end

http://simonecarletti.com/blog/2011/05/configuring-rails-3-https-ssl/