MongoMapper with ActiveModel - Rails 3

I'm trying out Rails 3 with MongoMapper for a new application, and have
run into a bug when both MongoMapper::Document and
ActiveModel::Validations are used in the same model, in this case
job.rb. I can create a new model with the various validations I require,
but when I attempt to save it I get:

NoMethodError: undefined method `_run_before_save_callbacks' for

Google doesn't come up with much for this other than suggestions to rely
only upon MongoMapper, which I would rather not do. So, if anyone can
suggest a workaround it would be very useful.