I saw this uninitialized constant ActiveRecord::TestFixtures error when i run the rake test. I have checked all the environment configuration. I am sure i am using Mongoid without ActiveRecord.
after reading the fixture_support.rb source code, I found out that the error is thrown only if ActiveRecord was called somewhere in my code. and it turned out to be caused by the Transition suggested by krzysiek: http://dev.netizer.pl/transitions-state-machine-for-rails-3.html
So, I have to switch the state machine to another gem which is not using ActiveRecord stuff. that's why i found stateflow: https://github.com/ryanza/stateflow
My test can run happily ever after.