sessions_controller.rb

# app/controllers/sessions_controller.rb

class SessionsController < ApplicationController
  # ...
  
  def create
    user = User.authenticate(params[:username], params[:password])
    
    if user
      session[:user_id] = user.id
      ActiveSupport::Notifications.instrument('user.logged_in', :username => user.name)
      redirect_to root_url, :notice => '...'
    else
      redirect_to login_url, :alert => '...'
    end
  end
end

user_transaction_logger.rb

# config/initializers/user_transaction_logger.rb

class FormattedLogger < Logger
  def format_message(severity, timestamp, progname, msg)
    "#{timestamp.strftime('%d.%m.%Y %H:%M:%S')} #{msg}\n"
  end
end

UserTransactionLogger = ActiveSupport::TaggedLogging.new(FormattedLogger.new(File.join(Rails.root, 'log', 'user_transaction.log')))

ActiveSupport::Notifications.subscribe('user.logged_in') do |name, start, finish, id, payload|
  UserTransactionLogger.tagged(payload[:username], 'User') do
    UserTransactionLogger.info 'logged in'
  end
end