authentication.rb

class Authentication < ActiveRecord::Base

  belongs_to :user

  def self.create_with_omniauth(auth_hash)
    create! do |auth|
      auth.provider = auth_hash["provider"]
      auth.uid = auth_hash["uid"]
      auth.token = auth_hash["credentials"]["token"]
    end
  end

  def update_token(auth_hash)
    self.token = auth_hash["credentials"]["token"]
    self.save
  end

end

Gemfile

gem 'omniauth'
gem 'omniauth-facebook'
gem 'clearance'

omniauth.rb

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :facebook, 'APP_KEY', 'APP_SECRET'
end

routes.rb

  match "/auth/:provider/callback" => "sessions#create_from_omniauth"

sessions_controller.rb

class SessionsController < Clearance::SessionsController

  def create_from_omniauth
    auth_hash = request.env["omniauth.auth"]

    authentication = Authentication.find_by_provider_and_uid(auth_hash["provider"], auth_hash["uid"]) || Authentication.create_with_omniauth(auth_hash)
    if authentication.user
      user = authentication.user 
      authentication.update_token(auth_hash)
      @next = root_url
      @notice = "Signed in!"
    else
      user = User.create_with_auth_and_hash(authentication,auth_hash)
      @next = edit_user_path(user)   
      @notice = "User created - confirm or edit details..."
    end
    sign_in(user)
    redirect_to @next, :notice => @notice
  end

end

user.rb

class User < ActiveRecord::Base

  include Clearance::User
 
  has_many :authentications, :dependent => :destroy
  
  def self.create_with_auth_and_hash(authentication,auth_hash)
    create! do |u|
      u.first_name = auth_hash["info"]["first_name"]
      u.last_name = auth_hash["info"]["last_name"]
      u.friendly_name = auth_hash["info"]["name"]
      u.email = auth_hash["extra"]["raw_info"]["email"]
      u.authentications<<(authentication)
    end
  end

  
  def fb_token
    x = self.authentications.where(:provider => :facebook).first
    return x.token unless x.nil?
  end
  
  
  def password_optional?
    true
  end

end