cart_controller.ex

  def add(conn, %{"product_id" => product_id}) do
    product = Front.get_product!(product_id)

    conn = case conn.assigns[:current_order] do
      nil ->
        order = Front.create_order_with_product!(product)
        conn |> put_session(:order_id, order.id)
      order ->
        Front.add_product_to_order!(order, product)
        conn
    end

    redirect(conn, to: cart_path(conn, :show))
  end