anonymous.ex

defmodule Plug.Anonymous do
	import Plug.Conn
  require IEx


  def init(options), do: options

  def call(conn, _opts) do
    anonymous_id = get_session(conn, "anonymous_id")
    case anonymous_id do
      nil -> put_session(conn, "anonymous_id", "Testing")
      _   -> conn
    end
  end
end

anonymous_plug_test.exs

defmodule Plug.AnonymousTest do
  use ExUnit.Case, async: true
  use Plug.Test
  alias Plug.Anonymous
  require IEx

  @default_opts [
    store: :cookie,
    key: "_test_key",
    encryption_salt: "encrypted salt",
    signing_salt: "signing salt",
    log: false
  ]

  @opts Plug.Session.init(@default_opts)

  @secret String.duplicate("abcdef0123456789", 8)

  defmodule SomeApp do
    defmodule SomeController do
      use Plug.Builder
      plug Plug.Session,
        store: :cookie,
        key: "_wankrank_key",
        signing_salt: "cmwAUEpB",
        max_age: 86400
      plug :fetch_session
      plug Anonymous
    end
  end

  def create_conn(conn, secret \\ @secret) do
    put_in(conn.secret_key_base, secret)
    |> Plug.Session.call(@opts)
    |> fetch_session
  end

  test "assigns user_id session key" do
    conn = conn(:get, "/") |> SomeApp.SomeController.call([])
		refute get_session(conn, "anonymous_id") == nil
  end

  test "returns existing user session key" do
      anonymous_id = %{anonymous_id: "some_id"}
      new_conn = create_conn(conn(:get, "/"))
      |> put_session("anonymous_id", "some_id")
      |> Plug.Anonymous.call([])
    assert get_session(new_conn, "anonymous_id") == "some_id"
  end

end