Creating user via API give correct responses if the user already exists or the mail address is malformed.

lib/beet_round_server_web/controllers/user_controller.ex

@@ -12,11 +12,23 @@ defmodule BeetRoundServerWeb.UserController do
   end
 
   def create(conn, %{"user" => user_params}) do
-    with {:ok, %User{} = user} <- Accounts.register_user(user_params) do
-      conn
-      |> put_status(:created)
-      |> put_resp_header("location", ~p"/api/users/#{user}")
-      |> render(:show, user: user)
+    case Accounts.register_user(user_params) do
+      {:ok, %User{} = user} ->
+        send_created_response(conn, user)
+
+      {:error, changeset} ->
+        with %User{} = user <- Accounts.get_user_by_email(user_params["email"]) do
+          send_already_reported_response(conn, user)
+        else
+          _ ->
+            send_not_acceptable(conn, changeset)
+        end
+
+      _ ->
+        conn
+        |> put_status(:bad_request)
+        |> put_resp_header("location", ~p"/api/users")
+        |> render(:show, changeset: "Bad request")
     end
   end
 
@@ -40,4 +52,24 @@ defmodule BeetRoundServerWeb.UserController do
   #     send_resp(conn, :no_content, "")
   #   end
   # end
+
+  defp send_created_response(conn, %User{} = user) do
+    conn
+    |> put_status(:created)
+    |> put_resp_header("location", ~p"/api/users/#{user}")
+    |> render(:show, %{user: user})
+  end
+
+  defp send_already_reported_response(conn, %User{} = user) do
+    conn
+    |> put_status(:already_reported)
+    |> render(:show, %{user: user})
+  end
+
+  defp send_not_acceptable(conn, changeset) do
+    conn
+    |> put_status(:not_acceptable)
+    |> put_resp_header("location", ~p"/api/users")
+    |> render(:show, changeset: changeset)
+  end
 end

lib/beet_round_server_web/controllers/user_json.ex

@@ -15,10 +15,31 @@ defmodule BeetRoundServerWeb.UserJSON do
     %{data: data(user)}
   end
 
+  def show(%{changeset: changeset}) do
+    # When encoded, the changeset returns its errors
+    # as a JSON object. So we just pass it forward.
+    %{errors: Ecto.Changeset.traverse_errors(changeset, &translate_error/1)}
+  end
+
   defp data(%User{} = user) do
     %{
       id: user.id,
       email: user.email
     }
   end
+
+  defp translate_error({msg, opts}) do
+    # You can make use of gettext to translate error messages by
+    # uncommenting and adjusting the following code:
+
+    # if count = opts[:count] do
+    #   Gettext.dngettext(BeetRoundServerWeb.Gettext, "errors", msg, msg, count, opts)
+    # else
+    #   Gettext.dgettext(BeetRoundServerWeb.Gettext, "errors", msg, opts)
+    # end
+
+    Enum.reduce(opts, msg, fn {key, value}, acc ->
+      String.replace(acc, "%{#{key}}", fn _ -> to_string(value) end)
+    end)
+  end
 end