Use the rails3 branch of MongoMapper, and add devise-mongo_mapper to your Gemfile
gem 'mongo_mapper', :git => 'git://github.com/jnunemaker/mongomapper', :branch => 'rails3' 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