Mongoid 3 is only compatible with Rails 3.x. Rails 4 requires Mongoid 4.
However, as of this writing, Mongoid 4 is still in development. I’ve had success using the master branch though.
1gem 'mongoid', github: 'mongoid/mongoid' 2 3# and if you use Sidekiq + Kiqstand: 4gem 'kiqstand', github: 'mongoid/kiqstand'