We can use ActiveRecord and Mongoid in one app. This Rails3 app is that sample.

Add 'bson_ext' and 'mongoid' to Gemfile, and run bundle. Then run rails g mongoid:config to generate a configuration file for Mongoid.

After installing mongoid, 'model' generator is for Mongoid.

$ r g model todo_log title:string done:boolean 
      invoke  mongoid
      create    app/models/todo_log.rb

So, when you're creating an ActiveRecord model file, use 'active_record:model'　generator.

$ r g active_record:model todo title:string done:boolean
      create  db/migrate/20110730071022_create_todos.rb
      create  app/models/todo.rb