def perform_update(self, serializer):
    # Save with the new value for the target model fields
    user = self.request.user
    userid = str(user.id)
    serializer.save(stu_enrolled_classes=userid)
# The above def is in viewset and you can specify what field else can be edited in the API "PUT",
# Here We just set the stu_enrolled_classes field with is relation to the user to be the current user that send the "PUT" request.