add oauth with google

app/controllers/users/omniauth_callbacks_controller.rb

@@ -0,0 +1,16 @@
+module Users
+  class OmniauthCallbacksController < Devise::OmniauthCallbacksController
+    def google_oauth2
+      # You need to implement the method below in your model (e.g. app/models/user.rb)
+      @user = User.find_by!(email: request.env['omniauth.auth'].info['email'])
+
+      flash[:notice] = I18n.t 'devise.omniauth_callbacks.success', kind: 'Google'
+      sign_in_and_redirect @user, event: :authentication
+    rescue ActiveRecord::RecordNotFound => e
+      # Removing extra as it can overflow some session stores
+      session['devise.google_data'] = request.env['omniauth.auth'].except('extra')
+
+      redirect_to new_user_registration_url, alert: e.message
+    end
+  end
+end