Django ModelForms：尝试使用外键ID保存表单

I'm trying to create a new Topic and the category id is dynamically determined in javascript on the client side. The problem i'm having is I pass the category id and I want to lookup the correct category object, but using a model form, it checks if the category is an instance of Category before I can assign it and save it.I'm trying to create a new Topic and the catego