Django - 未正确检索POST请求

I'm working on a inventory control in Django and i want a form to control the action that will be made. If a record for the product already exist, it should update the quantity. If it doesn't exist, it should create the record for it. Besides that, one form field will have three choices. One will add the form value to que product quantity, other will subtract and another one will change to the value in the form.I'm working on a inventory control in Django an