Add user ID to Rails logs with log tags and Authlogic

application.rb

MyApp::Application.configure do
  config.log_tags = [UserLogger.get]
end

user_logger.rb

# Change UserSession and User models, if needed.
module UserLogger
  extend self

  def get
    lambda { |req|
      return 0 if req.cookies[UserSession.cookie_key].nil?

      token = req.cookies[UserSession.cookie_key].match(/[A-Z0-9]+/i)[0]
      Rails.cache.fetch [UserSession.cookie_key, token] do
        user = User.where(persistence_token: token).pluck(:id)
        user || 0
      end
    }
  end
end