Phoenix Framework: PayPal IPN Handshake

IpnController.ex

defmodule AppName.IpnController do
  @moduledoc """
  IPN Controller.
  """

  use AppName.Web, :controller

  def create(conn, params) do
    # PayPal requires the params returned in the exact order given.
    # The map needs to be a list.  encode/1 keep the order of a list,
    # but not a map. https://hexdocs.pm/plug/Plug.Conn.Query.html.
    pp_query_string = Plug.Conn.Query.encode(
      params |> Enum.to_list
    )
    
    # Join the URI (e.g., sandbox/prod) with the params)
    pp_ack_uri = Enum.join(["https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_notify-validate", pp_query_string],"&")

    # Let it fly.
    case HTTPoison.post(pp_ack_uri,'',[],[]) do
      {:ok, response} ->
        IO.inspect(response)
        # Do business logic
      {:error, reason} ->
        IO.inspect(reason)
        # Uh, oh.
    end
    
    # Render a blank
    html(conn,'')
  end
end