Update the RedirectionController.

lib/link_shortener_web/controllers/redirection_controller.ex

@@ -0,0 +1,10 @@
+defmodule LinkShortenerWeb.RedirectionController do
+  use LinkShortenerWeb, :controller
+
+  alias LinkShortener.Links
+
+  def show(conn, %{"shorten" => shorten}) do
+    %{url: url} = Links.get_one_by_shorten!(shorten)
+    redirect(conn, external: url)
+  end
+end

lib/link_shortener_web/router.ex

@@ -18,6 +18,7 @@ defmodule LinkShortenerWeb.Router do
     pipe_through :browser
 
     get "/", PageController, :home
+    get "/u/:shorten", RedirectionController, :show
   end
 
   scope "/api", LinkShortenerWeb do