Rails logging is straightforward and works out of the box. But, sometimes I feel it would be really awesome if the log lines would contain the line number and file name which emit the log. If you have experience with log4j in Java, you should be able to recall that we initialize the logger and pass it a class instance. All our log lines are tagged with the classname. That’s cool for debugging purposes.

How to get that behavior in Rails? There is one quick and dirty way. Just prepend to the log message __LINE__ and __FILE__ which contain the line the code is executing and the file it is in, respectively.

logger.info("[#{__FILE__}:#{__LINE__}] The log message")

This just works. But shows the full file path which is kind of overkill because it produces very long log lines. The below one improves it to include just the filename.

logger.info("[#{__FILE__.split('/').last}:#{__LINE__}] The log message")

Ok, this gets the job done but makes our log statements ugly in the code. And it isn’t DRY. If we try to make it DRY, we can’t. Because we can’t assign the stuff on the left that produces file and line number strings, into a variable, because they will refer to the line where variable is assigned, so the log statement will contain wrong value for line number.

There is another way that involves ActiveSupport’s TaggedLogging. If you use a Tagged logger, you can associate a tag with every log message. That’s cool too, especially for searching through certain events in logs. TaggedLogging can be used to add file and line number, but the usage would be similar to the above example. How about doing something that works seamlessly behind the scenes and is DRY?

After some research, I wrote a class (ok, I assembled it from here and there), whose instance can be assigned to Rails log formatter. Here it is. Just put this class in your main application module and assign it’s instance to rails log formatter. Apart from filename and line number, I have also edited the log line to have some other stuff displayed in fancy colors. Also, I don’t print filename and line if the filename is logger.rb or starts with log_. That’s because it would create so much spam due to Rails internal log files or files from various gems. Use it and change it to your liking.

module ApplicationName

  class QLogFormatter
    HOSTNAME = Socket.gethostname
      'DEBUG' => '35',
      'INFO' => '32',
      'WARN' => '33',
      'ERROR' => '31',
      'FATAL' => '31',
      'UNKNOWN' => '37'
    def call(severity, time, progname, msg)
      return "" if (msg.blank? || msg.strip.blank?)
      formatted_time = time.strftime("%Y-%m-%d %H:%M:%S.") << time.usec.to_s[0..2]
      color = SEVERITY_TO_COLOR_MAP[severity]
      callee = caller[5].split('/').last
      callee = callee.split(':')[0,2].join(':')
      log_line = "\033[0;37m#{HOSTNAME}@#{formatted_time}\033[0m [\033[01;#{color}m#{severity}\033[0m]"
      unless (callee.start_with?('logger') || callee.start_with?('log_'))
        log_line += "[\033[01;36m#{callee}\033[0m]"
      log_line += " #{msg}\n"

  class Application < Rails::Application
    # All your other configs
    config.log_formatter = QLogFormatter.new