mastodon/app/channels/application_cable/connection.rb

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

    def connect
      self.current_user = find_verified_user
    end

    protected

    def find_verified_user
      verified_user = env['warden'].user

      if verified_user
        verified_user
      else
        reject_unauthorized_connection
      end
    end
  end
end