Rails 3: how to use active record and mongoid at the same time
Well, to do it, you're supposed to leave Rails intact, so don't exclude libraries like it's commonly suggested in Mongoid documentation. You need to configure them separately, so you need to have a database.yml and mongo.yml config files and you need to make sure they're both getting loaded.
After that, you can enable/disable AR and Mongoid on a per-model basis.
class User < ActiveRecord::Base #this is an AR modelendclass Item include Mongoid::Document #this is a Mongoid modelend