rails/actionpack/lib/action_controller/log_subscriber.rb

require 'active_support/core_ext/object/blank'

module ActionController
  class LogSubscriber < ActiveSupport::LogSubscriber
    INTERNAL_PARAMS = %w(controller action format _method only_path)

    def start_processing(event)
      payload = event.payload
      params  = payload[:params].except(*INTERNAL_PARAMS)
      format  = payload[:format]
      format  = format.to_s.upcase if format.is_a?(Symbol)

      info "Processing by #{payload[:controller]}##{payload[:action]} as #{format}"
      info "  Parameters: #{params.inspect}" unless params.empty?
    end

    def process_action(event)
      payload   = event.payload
      additions = ActionController::Base.log_process_action(payload)

      status = payload[:status]
      if status.nil? && payload[:exception].present?
        exception_class_name = payload[:exception].first
        status = ActionDispatch::ExceptionWrapper.status_code_for_exception(exception_class_name)
      end
      message = "Completed #{status} #{Rack::Utils::HTTP_STATUS_CODES[status]} in #{format_duration(event.duration)}"
      message << " (#{additions.join(" | ")})" unless additions.blank?

      info(message)
    end

    def halted_callback(event)
      info "Filter chain halted as #{event.payload[:filter]} rendered or redirected"
    end

    def send_file(event)
      info("Sent file #{event.payload[:path]} (#{format_duration(event.duration)})")
    end

    def redirect_to(event)
      info "Redirected to #{event.payload[:location]}"
    end

    def send_data(event)
      info("Sent data #{event.payload[:filename]}  (#{format_duration(event.duration)})")
    end

    %w(write_fragment read_fragment exist_fragment?
       expire_fragment expire_page write_page).each do |method|
      class_eval <<-METHOD, __FILE__, __LINE__ + 1
        def #{method}(event)
          key_or_path = event.payload[:key] || event.payload[:path]
          human_name  = #{method.to_s.humanize.inspect}
          duration = format_duration(event.duration)
          info("\#{human_name} \#{key_or_path} \#{duration}")
        end
      METHOD
    end

    def logger
      ActionController::Base.logger
    end
  end
end

ActionController::LogSubscriber.attach_to :action_controller