authenticator.ex

defmodule MyApp.Authenticator do
  import Plug.Conn

  def attempt_to_authenticate(conn, _) do
    case find_account(conn) do
      { :ok, account } ->
        assign(conn, :account, account)
      :error ->
        conn
          |> send_resp(401, "")
          |> halt
    end
  end

  defp find_account(conn) do
    account_id = conn
    |> fetch_session
    |> get_session(:account_id)

    case MyApp.Repo.get(MyApp.Account, account_id) do
      nil ->
        :error
      account ->
        { :ok, account }
    end
  end
end

router.ex

defmodule MyApp.Route do
  pipeline :authenticated do
    plug :authenticated
  end
end