My own logger formatter for Rails dev env.

development_logger_formatter.rb

# Custom logger formatter for development environment.
#
# Examples
#
#   # in config/environments/development.rb
#   config.log_formatter = ::DevelopmentLoggerFormatter.new
#
#   # in controller
#   logger.error(self.class) { 'This is a error message' }
#   logger.info { 'This is a info message' }
#
# Output:
#
#   2015/08/10 20:12:05 [ERROR] HomeController -- This is a error message
#   2015/08/10 20:12:05 [INFO ] This is a info message
#
class DevelopmentLoggerFormatter
  def call(severity, timestamp, progname, msg)
    format("%s [%-5s] %s%s\n",
           timestamp.strftime('%Y/%m/%d %H:%M:%S'),
           severity,
           progname ? "#{progname} -- " : nil,
           msg)
  end
end