"Elixir + phoenix" filtering websocket messages - websocket

Question

I try to preprocess websocket incoming messages. In lobby_channel.ex file I have:
defmodule Chatroom.LobbyChannel do
use Phoenix.Channel
def join("room:lobby", _payload, socket) do
{:ok, socket}
end
def join("room:" <> _private_room_id, _params, _socket) do
{:error, %{reason: "unauthorized"}}
end
def handle_in("new_move", payload, socket) do
changeset = Chatroom.Move.changeset(%Chatroom.Move{}, payload)
if changeset.valid? do
broadcast! socket, "new_move", payload
{:noreply, socket}
else
IO.puts "-----" <> changeset.errors[:message]
push socket, "error", %{msg: "Invalid move"}
{:noreply, socket}
end
end
#intercept ["new_move"]
#def handle_out("new_move", payload, socket) do
# IO.puts payload["name"] <> "=======" <> payload["message"]
# push socket, "new_move", payload
# {:noreply, socket}
#end
end
Model file contains validation functions:
...
def changeset(struct, params \\ %{}) do
struct
|> cast(params, [:name, :message])
|> validate_required([:name, :message])
|> validate_length(:name, min: 1)
|> validate_length(:message, is: 2)
|> is_move_allowed(params)
end
defp is_move_allowed(changeset, params) do
IO.puts "--zz---" <> params["message"]
if params["message"] == "00" do
IO.puts "--yy---" <> params["message"]
add_error(changeset, params["message"], "'message' cannot be '00'")
else
IO.puts "--xx---" <> params["message"]
[]
end
end
...
..the last one is my custom validation function and I can't make it working without errors. The output I get:
--zz---00
--yy---00
[error] GenServer #PID<0.445.0> terminating
** (ArgumentError) argument error
:erlang.byte_size(nil)
(chatroom) web/channels/lobby_channel.ex:18: Chatroom.LobbyChannel.handle_in/3
(phoenix) lib/phoenix/channel/server.ex:226: anonymous fn/4 in Phoenix.Channel.Server.handle_info/2
(stdlib) gen_server.erl:601: :gen_server.try_dispatch/4
(stdlib) gen_server.erl:667: :gen_server.handle_msg/5
(stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: %Phoenix.Socket.Message{event: "new_move", payload: %{"message" => "00", "name" => "a"}, ref: "2", to
pic: "room:lobby"}
State: %Phoenix.Socket{assigns: %{}, channel: Chatroom.LobbyChannel, channel_pid: #PID<0.445.0>, endpoint: Chatroom
.Endpoint, handler: Chatroom.UserSocket, id: nil, joined: true, pubsub_server: Chatroom.PubSub, ref: nil, serialize
r: Phoenix.Transports.WebSocketSerializer, topic: "room:lobby", transport: Phoenix.Transports.WebSocket, transport_
name: :websocket, transport_pid: #PID<0.439.0>}
As I understand, things are moving to the right way logically, message contains correct data, but how can I fix the error? What am I missing?

Answer 1

There are three errors in the code:
is_move_allowed must always return a Changeset. You're returning [] in else.
add_error's second argument should be an atom
changeset.errors[:message] is a tuple, so you cannot use <> to concatenate it with a String. IO.inspect is a better alternative as it'll print any value.
So, change:
if params["message"] == "00" do
IO.puts "--yy---" <> params["message"]
add_error(changeset, params["message"], "'message' cannot be '00'")
else
IO.puts "--xx---" <> params["message"]
[]
end
to:
if params["message"] == "00" do
IO.puts "--yy---" <> params["message"]
add_error(changeset, :message, "'message' cannot be '00'")
else
IO.puts "--xx---" <> params["message"]
changeset
end
and change:
IO.puts "-----" <> changeset.errors[:message]
to:
IO.inspect changeset.errors[:message]
# or IO.inspect changeset.errors