Make /u/<username> work same way as /u/<id>

lib/asciinema/accounts.ex

@@ -20,6 +20,14 @@ defmodule Asciinema.Accounts do
 
   def get_user(id), do: Repo.get(User, id)
 
+  def find_user_by_username(username) do
+    Repo.one(
+      from(u in User,
+        where: fragment("lower(?)", u.username) == ^String.downcase(username)
+      )
+    )
+  end
+
   def find_user_by_auth_token(auth_token) do
     Repo.get_by(User, auth_token: auth_token)
   end
@@ -120,18 +128,11 @@ defmodule Asciinema.Accounts do
     end
   end
 
-  def lookup_user(identifier) do
+  def lookup_user(identifier) when is_binary(identifier) do
     if String.contains?(identifier, "@") do
       {:email, Repo.get_by(User, email: identifier)}
     else
-      user =
-        Repo.one(
-          from(u in User,
-            where: fragment("lower(?)", u.username) == ^String.downcase(identifier)
-          )
-        )
-
-      {:username, user}
+      {:username, find_user_by_username(identifier)}
     end
   end
 

lib/asciinema_web/controllers/user_controller.ex

@@ -47,8 +47,10 @@ defmodule AsciinemaWeb.UserController do
   end
 
   def show(conn, params) do
-    with {:ok, user} <- fetch_user(params) do
+    if user = fetch_user(params) do
       do_show(conn, params, user)
+    else
+      {:error, :not_found}
     end
   end
 
@@ -84,14 +86,15 @@ defmodule AsciinemaWeb.UserController do
   end
 
   defp fetch_user(%{"id" => id}) do
-    case Integer.parse(id) do
-      {id, ""} -> Accounts.fetch_user(id)
-      _ -> {:error, :not_found}
+    if String.match?(id, ~r/^\d+$/) do
+      Accounts.get_user(id)
+    else
+      Accounts.find_user_by_username(id)
     end
   end
 
   defp fetch_user(%{"username" => username}) do
-    Accounts.fetch_user(username: username)
+    Accounts.find_user_by_username(username)
   end
 
   def edit(conn, _params) do