# Upgrade Rails 2.1.2 -> 2.3.4
1) # environment.rb
RAILS_GEM_VERSION = '2.3.4' unless defined? RAILS_GEM_VERSION
2) rake rails:upgrade
3) ruby script/server in rails app home directory in development mode leads to:

ArgumentError (A copy of UserController has been removed from the module tree but is still active!):
  /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.4/lib/active_support/dependencies.rb:414:in `load_missing_constant'
  /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.4/lib/active_support/dependencies.rb:96:in `const_missing'
  app/controllers/user_controller.rb:269:in `current_user'
  app/controllers/user_controller.rb:261:in `logged_in?'
  haml (2.2.3) lib/sass/plugin/rails.rb:19:in `process'