ruby erb 37 lines · 3 tabs

Presence indicator with ActionCable + Turbo Streams

Henry Kim Jan 2026

3 tabs

module ApplicationCable
  class Connection < ActionCable::Connection::Base
    identified_by :current_member

    def connect
      self.current_member = env['warden']&.user || reject_unauthorized_connection
    end
  end
end

class RoomChannel < ApplicationCable::Channel
  def subscribed
    @room = Room.find(params[:room_id])
    stream_for @room

    Presence.track_join(@room.id, current_member.id)
    broadcast_presence
  end

  def unsubscribed
    Presence.track_leave(@room.id, current_member.id)
    broadcast_presence
  end

  private

  def broadcast_presence
    Turbo::StreamsChannel.broadcast_replace_to(
      @room,
      target: 'room_presence',
      partial: 'rooms/presence',
      locals: { room: @room, member_ids: Presence.online_member_ids(@room.id) }
    )
  end
end

<div id="room_presence" class="text-sm text-gray-600">
  Online: <%= Member.where(id: member_ids).pluck(:nickname).join(', ') %>
</div>

3 files · ruby, erb Explain with highlit

Presence is usually overkill, but for collaboration features it’s valuable: show who’s online in a room. I identify connections with current_member in ApplicationCable::Connection, then in a channel I broadcast updates when members subscribe/unsubscribe. The UI subscribes with turbo_stream_from @room and renders a partial listing online members. I keep the data minimal and ephemeral: store online member IDs in Redis or a simple in-memory store (depending on deployment), and never treat presence as security. The Turbo approach is nice because updates are HTML fragments, so styling is consistent and you don’t build a JSON protocol. The main gotcha is scale: presence can be chatty, so throttle broadcasts and keep the partial fast.

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