app/controllers/users/omniauth_callbacks_controller.rb
class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
def google_oauth2
# You need to implement the method below in your model (e.g. app/models/user.rb)
@user = User.from_omniauth(request.env['omniauth.auth'], current_user)
raise 'Failed to get persisted user from omniauth data' unless @user.persisted?
flash[:notice] = I18n.t 'devise.omniauth_callbacks.success', :kind => 'Google'
sign_in_and_redirect @user, :event => :authentication
end
end