Update the create link route to add the relationship to the user.

lib/link_shortener_web/controllers/api/v1/link_controller.ex

@@ -12,6 +12,12 @@ defmodule LinkShortenerWeb.Api.V1.LinkController do
   end
 
   def create(conn, %{"link" => link_params}) do
+    user = Guardian.Plug.current_resource(conn)
+
+    link_params =
+      link_params
+      |> Map.put("user_id", user.id)
+
     with {:ok, %Link{} = link} <- Links.insert_one(link_params) do
       conn
       |> put_status(:created)