Notifiable concern

app/models/concerns/notifiable.rb

+# == Notifiable concern
+#
+# Contains notification functionality shared between UsersProject and UsersGroup
+#
+module Notifiable
+  extend ActiveSupport::Concern
+
+  included do
+    validates :notification_level, inclusion: { in: Notification.project_notification_levels }, presence: true
+  end
+
+  def notification
+    @notification ||= Notification.new(self)
+  end
+end