Logging in JSON with Ruby/Rails

json_logger.rb

require 'logger'
require 'time'
require 'json'
require 'forwardable'

# A simple subclass that writes JSON logs
class JsonLogger < Logger
  def format_message(severity, timestamp, progname, msg)
    msg = {message: msg} unless msg.is_a? Hash
    log = {timestamp: timestamp.iso8601, severity: severity}.merge(msg)
    "#{log.to_json}\n"
  end
end

# An example AuditLogger for a Rails app
class AuditLogger
  extend SingleForwardable
  @logger = JsonLogger.new "#{Rails.root}/log/audit.log"
  delegate [:debug, :info, :warn, :error, :fatal] => :@logger
end

# Example usage:
# AuditLogger.info(
#   event: "UserSignedUp",
#   user: "abc@def.com",
# )