WebSockets: Can connect but cannot subscribe to channel - ruby-on-rails

Question

I have a rails API-only app running in nginx with passenger. I'm trying to use Action Cable to set up a simple chat application in Flutter but I can't seem to get it to work correctly.
I am able to connect using the following:
channel = IOWebSocketChannel.connect("wss://my.domain/cable")
and see the following in my production.log when that happens:
Started GET "/cable" for [redacted] at 2019-03-14 19:44:09 +0000
Started GET "/cable" [WebSocket] for [redacted] at 2019-03-14 19:44:09 +0000
Successfully upgraded to WebSocket (REQUEST_METHOD: GET, HTTP_CONNECTION: upgrade, HTTP_UPGRADE: websocket)
Doorkeeper::AccessToken Load (0.3ms) SELECT `oauth_access_tokens`.* FROM `oauth_access_tokens` WHERE `oauth_access_tokens`.`token` = '[redacted]' LIMIT 1
User Load (0.2ms) SELECT `users`.* FROM `users` WHERE `users`.`id` = 2 LIMIT 1
Registered connection (Z2lkOi8vYWdlbnQyNHg3L1VzZXIvMg)
So the connection seems to be made and successfully upgraded
I am then sending a subscribe command
channel.sink.add(json.encode({
"command": "subscribe",
"identifier": {"channel": "ConversationChannel", "conversation_id": "${conversation.id}"}
}));
I'm not sure if I should be seeing anything in the log files after this command is executed, but I don't. Nothing happens and there is no indication of any error in my production.log or my nginx error log. I set up some logging in the subscribe method of the ConversationChannel so it would at least output something to the production log when it is called, so I could test if it's at least even calling the subscribe command
class ConversationChannel < ApplicationCable::Channel
def subscribed
Rails.logger.info "test conversation subscribe"
reject && return unless current_user
stream_from "conversation_#{params[:conversation_id]}_channel"
end
...
end
but this does not appear in production.log so it would seem that the subscription command is not getting to where it is supposed to go.
I then tried using dart WebSocket using the code located here, which came from a conversation on github regarding connection states, to see if I could determine if the connection was coming back okay.
When I run that code with my domain I see the same successful connection in the production.log, but the connection actually times out with onerror TimeoutException after 0:00:15.000000: Future not completed which seems to indicate that the connection actually doesn't succeed after all, despite what it says in the log file.
What am I doing wrong here?

Answer 1

If I remember, when you're attempting to connect to a channel, your identifier has to be JSON stringified, as well as your whole JSON object. Take a look at this link:
https://thoughtbot.com/blog/talking-to-actioncable-without-rails
In particular:
const msg = {
command: 'subscribe',
identifier: JSON.stringify({
channel: 'SomeChannel',
}),
};
socket.send(JSON.stringify(msg));
Rails does the whole JSON parsing in the backend. See if this helps you out. I've had similar issues like this before, when attempting to connect the WebSockets, without the usage of their ActionCable npm library.