ActiveSupport::LogSubscriber is an object set to consume ActiveSupport::Notifications with the sole purpose of logging them. The log subscriber dispatches notifications to a registered object based on its given namespace.

An example would be Active Record log subscriber responsible for logging queries:

module ActiveRecord
  class LogSubscriber < ActiveSupport::LogSubscriber
    def sql(event)
      "#{event.payload[:name]} (#{event.duration}) #{event.payload[:sql]}"
    end
  end
end

And it's finally registered as:

ActiveRecord::LogSubscriber.attach_to :active_record

Since we need to know all instance methods before attaching the log subscriber, the line above should be called after your ActiveRecord::LogSubscriber definition.

After configured, whenever a “sql.active_record” notification is published, it will properly dispatch the event (ActiveSupport::Notifications::Event) to the sql method.

Log subscriber also has some helpers to deal with logging and automatically flushes all logs when the request finishes (via action_dispatch.callback notification) in a Rails environment.

Namespace

Module

Methods

Constants

BLACK = "\e[30m"
 

Colors
BLUE = "\e[34m"
BOLD = "\e[1m"
CLEAR = "\e[0m"
 

Embed in a String to clear all previous ANSI sequences.
CYAN = "\e[36m"
GREEN = "\e[32m"
MAGENTA = "\e[35m"
RED = "\e[31m"
WHITE = "\e[37m"
YELLOW = "\e[33m"

Attributes

[W] logger

Class Public methods

flush_all!()

Flush all log_subscribers' logger.

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 70
      def flush_all!
        logger.flush if logger.respond_to?(:flush)
      end
🔎 See on GitHub

log_subscribers()

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 65
      def log_subscribers
        subscribers
      end
🔎 See on GitHub

logger()

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 57
      def logger
        @logger ||= if defined?(Rails) && Rails.respond_to?(:logger)
          Rails.logger
        end
      end
🔎 See on GitHub

Instance Public methods

finish(name, id, payload)

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 83
    def finish(name, id, payload)
      super if logger
    rescue => e
      if logger
        logger.error "Could not log #{name.inspect} event. #{e.class}: #{e.message} #{e.backtrace}"
      end
    end
🔎 See on GitHub

logger()

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 75
    def logger
      LogSubscriber.logger
    end
🔎 See on GitHub

start(name, id, payload)

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 79
    def start(name, id, payload)
      super if logger
    end
🔎 See on GitHub

Instance Private methods

color(text, color, bold = false)

Set color by using a symbol or one of the defined constants. If a third option is set to true, it also adds bold to the string. This is based on the Highline implementation and will automatically append CLEAR to the end of the returned String.

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 105
    def color(text, color, bold = false) # :doc:
      return text unless colorize_logging
      color = self.class.const_get(color.upcase) if color.is_a?(Symbol)
      bold  = bold ? BOLD : ""
      "#{bold}#{color}#{text}#{CLEAR}"
    end
🔎 See on GitHub