How to detect current topic?

Question

Where would I store topic id?

As for socket, I can use:

def join("topic:" <> topic_id, _params, socket) do
    ...
    socket= assign(socket, :topic_id, topic_id)
    {:ok, socket}
end

That was at socket scope, but my users can join multiple topics at the same time, meaning that the above code will override the topic_id each time a new topic is joined, is that true?

What if I want to know which topic id is active in the handle_in ?

for example:

def handle_in("new_message", params, socket) do

    # what is the active topic id here?

end

I though of this:

def handle_in("new_message:" <> topic_id, params, socket) do

    # now, I know that topic_id is the active topic

end

Is there another way to do this? or this is how its done?

Answer 1

The join has a topic so that you can perform additional validation to check the user can subscribe to the topic (check their permissions, etc.)

You are correct, after there is a subscription to the topic, the channels are multiplexed over the socket.

If you wish to pass additional information for a particular message, the params is a common place to put them:

def handle_in("new_message", %{"topic_id" => topic_id}, socket) do
  ...
end

If you could explain why you need the topic_id then it could help answer your questy,