Remember that all data structures in Elixir are immutable, so is a %Plug.Conn{}.

Plug.Conn.assign/3 always returns a new %Plug.Conn{}. So your code should be

conn = conn
       |> put_session(:user_id, user.id)
       |> configure_session(renew: true)
       |> put_flash(:info, "Congratulations, #{user.first_name}, you were signed in successfully.")
       |> assign(:current_user, user)

IEx.pry

redirect(conn, to: page_path(conn, :index))

Note that the local variable conn is reassigned.

I’m not familiar with Guardian so excuse me if I’m wrong.

Since you’re using token authentication, the token should be maintained by the client side code (e.g. javascript on the browser side). The client should carry the token on every request whenever the token is available.

However, redirect is a two-step action:

1. the browser receives a response with status code 302 and a Location header, which contains a URL.
2. the browser sends another GET request immediately to the URL in Location header.

Unfortunately, you can’t control the second step. You can’t add headers to it, you can’t specify the HTTP method, you can’t add cookie to it, and you can’t attach token to it. I think this is the reason why you lose the current_user after redirect.

If you are building a single-page application, try not to use redirect. Let Javascript handle page switching.