看这里有解决方案： ( see this post: http://stackoverflow.com/questions/6372626/using-active-record-generators-after-mongoid-installation)
$ rails g active_record:migration
之所以会这样，是因为mongoid 一样会调用 rails g生成mongoid 需要的ORM MODEL. ( this is caused by mongoid's ORM strategy which would generate a model for mongoid but not ActiveRecord )