Have you tried RSpec + Mongoid + DatabaseCleaner without using ActiveRecord? I'm concerned about these:

* https://github.com/fnando/factory_girl-preload/blob/master/lib/factory_girl/preload.rb#L37-L51
* https://github.com/fnando/factory_girl-preload/blob/master/lib/factory_girl/preload/rspec2.rb#L6