how to update ManyToManyField while an instance been created

lookup problem. mobile lookup to MobileStorage, then mobilecustomer lookup to MobileCustomer.

def form_valid(self, form):
  # self.object = None
  super().form_valid(form)
  # self.object = a instance of MobileCustomer
  imei = Imei.objects.filter(mobile__mobilecustomer=self.object).update(active=False)
  print(imei)