config/routes.rb

@@ -59,3 +59,9 @@ Redstoner::Application.routes.draw do
 
   root to: 'statics#index'
 end
+
+constraints(host: /^(?!www\.)/i) do
+  match '(*any)' => redirect { |params, request|
+    URI.parse(request.url).tap { |uri| uri.host = "www.#{uri.host}" }.to_s
+  }
+end