I added the following plug to my application.

For anyone willing to use this, a few pointers. The log statements are very useful for debugging and understanding whats going on in your app but they will slow down your app because they happen on every request. So remove them for serious use.

The @invalid_return_to is added so the return_to variable does not get overwritten during your authentication process. The exact values depend on the authentication system you are using. So you might need to change them to work with your app.

defmodule MyAppWeb.ReturnToPlug do
  import Plug.Conn

  @invalid_return_to ["auth", "sign-in", "sign-out"]

  def init(default), do: default

  def call(conn, _default) do
    IO.puts("""
    Verb: #{inspect(conn.method)}
    Host: #{inspect(conn.query_string)}
    Headers: #{inspect(conn.request_path)}
    session: #{inspect(get_session(conn))}
    """)

    conn.request_path
    |> is_invalid_return_to()
    |> if do
      conn
    else
      put_session(conn, :return_to, add_query_parameter(conn.request_path, conn.query_string))
    end
  end

  defp is_invalid_return_to(path) do
    @invalid_return_to
    |> Enum.map(fn invalid -> String.contains?(path, invalid) end)
    |> Enum.any?()
  end

  defp add_query_parameter(path, query) do
    if query == "" do
      path
    else
      "#{path}?#{query}"
    end
  end
end

And in the router add:

    plug MyAppWeb.ReturnToPlug

You are welcome to use it, ;). I did not do a lot of testing yet so there might be a few bugs hiding.

I did a bit more testing and I noticed that when i signed out it keeps returning me to the login screen. This is because the auth_controller of AshAuthenticationPhoenix also makes use of the return_to value. In my case I just want it to return me to my homepage so with a small edit to the controller this was easily solved.

    return_to = g̵e̵t̵_̵s̵e̵s̵s̵i̵o̵n̵(̵c̵o̵n̵n̵,̵ ̵:̵r̵e̵t̵u̵r̵n̵_̵t̵o̵)̵ ̵|̵|̵ ~p"/"

Another solution might be to prevent setting the return_to variable when we have a logged in user.

@zachdaniel Would you like me to create an issue against AshAuthenticationPhoenix for either of these versions?

Seems like a great candidate for both a standalone igniter task for devs to use to augment their installs.