http://blog.guiltry.com/devise-multiple-provider-with-factory-method-pattern

user_factory.rb

# factory_method/user.rb
module FactoryMethod
  # [Role] Factory
  class User
    def self.create(type, data=nil, current_user=nil)
      
      if type == 'normal'
        # login normally
        
      else 
        # OAUTH

        # Get the identity and user if they exist
        identity = Identity.find_or_create_by(uid: data.uid, provider: data.provider)

        # If a current_user is provided it always overrides the existing user
        # to prevent the identity being locked with accidentally created accounts.
        # Note that this may leave zombie accounts (with no associated identity) which
        # can be cleaned up at a later date.
        @user = current_user || identity.user

        # Create the user if needed
        if @user.blank?
          case type
            when 'google_oauth2'
              @user = GoogleUser.new(identity, data).create
            when 'facebook'
              @user = FacebookUser.new(identity, data).create
          end
        end
      end
      @user
    end
  end
end