许多tomany字段的Django-admin验证

I have a django model where I have a Module class which can have a field 'sort' set to one of 4 values (Video, Article, Quiz, Test). If Video or Article are selected then I have validation in place to ensure the field 'textfile' (which is an uploaded file) is not null. I want validation that if Quiz or Test is selected then at least one question is also added. You can see my attempted validation in the class Module below but it doesn't work.I have a django model where I have a Module cla