Read the full article "Omniauth pure: Authentication with Facebook, Google, Google Apps, Twitter, Github, AOL, MyOpenID and many other OpenID providers" on CommunityGuides: 
http://www.communityguides.eu/articles/16

This article will demonstrate how to set up a multi-provider authentication using the fabulous Omniauth gem. Users will be able to add multiple providers to their account and views for sign-in, sign-up and management of linked accounts are provided. The full source code is available on Github as a basis for your own projects.

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Credits:
Omniauth: http://github.com/intridea/omniauth
Omniauth Buttons: http://github.com/intridea/authbuttons


If you want to use Omniauth pure as a starting point in one of your applications, you just need to:
* add Omniauth to your gems
* generate a user model with whatever fields you like: rails generate model user name:string ...
* generate the service model: rails generate model service user_id:integer provider:string uid:string uname:string uemail:string
* generate the service controller: rails generate controller services
* set the secret for the cookie store in config/initializers/session_store!
* copy part of routes.rb
* copy the models service.rb and user.rb (or edit, if you already have a user model!)
* copy the code of the services_controller
* copy all views from services
* copy (parts of) the stylesheet
* and refer to the article for details...