This is a working example of combining Devise, Cancan, MongoDB, and OmniAuth together in a Rails 3.2.2 application. It installs the 10gen MongoDB packages, since they are generally fresher than those in the Debian or Ubuntu repositories.

Usage:

From the 10gen MongoDB Guide - Installing Ubuntu and Debian Packages

  sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 7F0CEB10  
  sudo nano /etc/apt/sources.list  

Add the line, then save the file.

  deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen  
  sudo apt-get update  
  sudo apt-get install mongodb-10gen  

FOR UBUNTU EARLIER THAN 11.04

Install mongodb according to this guide then

  apt-get install libxml2-dev libxslt-dev  

The next step is to go into the application's /config/initializers directory and create a new file. We'll call it omniauth.rb but the name isn't really important. In this file we'll add OmniAuth::Builder to our application's middleware and define the providers that our application will use to authenticate against.

See the example omniauth configuration file '/config/initializers/omniauth.rb.example'

   Rails.application.config.middleware.use OmniAuth::Builder do  
     provider :twitter, 'CONSUMER_KEY', 'CONSUMER_SECRET'  
     provider :facebook, 'APP_ID', 'APP_SECRET'  
     provider :linked_in, 'CONSUMER_KEY', 'CONSUMER_SECRET'  
   end

   cp ./config/initializers/devise.rb.example ./config/initializers/devise.rb

Modify the file to your liking.

  bundle install
  rake db:create
  rake db:migrate