Add MongoMapper and devise-mongo_mapper to your Gemfile
gem 'mongo_mapper'
gem 'devise-mongo_mapper',
:git => 'git://github.com/collectiveidea/devise-mongo_mapper
Comment out the current ORM in config/initializers/devise.rb, and add:
require 'devise/orm/mongo_mapper'
Configure your model:
class User
include MongoMapper::Document
plugin MongoMapper::Devise
devise :registerable, :database_authenticatable, :recoverable
end