Example on how to extract Devise user ID for Rails TaggedLogger.

config-application.rb

    config.log_tags = [
      :remote_ip,
      ->(req){
        if user_id = WardenTaggedLogger.extract_user_id_from_request(req)
          "#" + user_id.to_s
        else
          "guest"
        end
      }
    ]

lib-warden_tagged_logger.rb

module WardenTaggedLogger
  def self.extract_user_id_from_request(req)
    session_key = Rails.application.config.session_options[:key]
    session_data = req.cookie_jar.encrypted[session_key]
    warden_data = session_data["warden.user.user.key"]
    warden_data[0][0]
    rescue
      nil
  end
end

This hack was sponsored by:
BrideZilla - Track Your Wedding Photography Clients & Be In Control

This is a solution for: http://stackoverflow.com/questions/10811393/how-to-log-user-name-in-rails/13819155#13819155

;-) cool

Never use begin rescue in a critical path like this one:

require 'benchmark/ips'

good_hash = {
    'warden.user.admin_user.key' => [[1]]
}

bad_hash = {
}

Benchmark.ips do |x|
  x.report('begin+rescue good hash') { good_hash['warden.user.admin_user.key'][0][0] }
  x.report('has_key good hash') do
    good_hash.has_key?('warden.user.admin_user.key') &&
        good_hash['warden.user.admin_user.key'][0][0]
  end

  x.report('begin+rescue bad hash') { bad_hash['warden.user.admin_user.key'][0][0] rescue nil }
  x.report('has_key bad hash') do
    if bad_hash.has_key?('warden.user.admin_user.key')
      bad_hash['warden.user.admin_user.key'][0][0]
    end
  end

  # Compare the iterations per second of the various reports!
  x.compare!
end

Comparison:
    has_key bad hash:  4870164.1 i/s
begin+rescue good hash:  3544134.7 i/s - 1.37x slower
   has_key good hash:  2127500.6 i/s - 2.29x slower
begin+rescue bad hash:     4468.2 i/s - 1089.95x slower