osem/app/controllers/users/omniauth_callbacks_controller.rb

module Users
  class OmniauthCallbacksController < Devise::OmniauthCallbacksController
    skip_before_filter :verify_authenticity_token
    skip_authorization_check

    User.omniauth_providers.each do |provider|
      define_method(provider) { handle(provider) }
    end

    private

    def handle(provider)
      auth_hash = request.env['omniauth.auth']
      username = auth_hash.info.email.split('@')[0]
      openid = Openid.find_for_oauth(auth_hash) # Get or create openid
      # If openid exists and is associated with a user, sign in with associated user,
      # even if the email of the associated user and the email of the provided openid are different
      unless (user = openid.user)
        user = User.find_for_auth(auth_hash, current_user) # Get or create users
        user.username = "#{username}@#{provider}" if user.username.blank?
      end

      begin
        user.save!
        if openid.user != user
          openid.user = user
        end
        openid.save!

        sign_in user
        redirect_to request.env['omniauth.origin'] || root_path,
                    notice: "#{user.email} signed in successfully with #{provider}"
      rescue => e
        flash[:error] = e.message
        redirect_back_or_to new_user_registration_path
      end
    end
  end
end