Python 在不使用django rest框架中的信号的情况下，如何在更新时获取django模型字段的旧值？

我正在使用django rest框架。我有一个案例，在创建或更新第一个模型的实例后，我需要更新其他模型的字段

这是我的模型。py:

class FirstModel(models.Model):
    first_model_item = models.ForeignKey(Item)
    quantity =  models.IntegerField()

class SecondModel(models.Model):
    second_model_item = models.ForeignKey(FirstModel)
    quantity = models.IntegerField()

class CreateSecondModelView(generics.CreateAPIView):
    permission_classes = ()
    queryset = SecondModel.objects.all()
    serializer_class = SecondModeSerializer

    def perform_create(self, serializer):
        instance = serializer.save()
        first_model_item = instance.item
        first_model_item.quantity -= instance.quantity
        first_model_item.save()


class UpdateSecondModelView(generics.RetrieveUpdateAPIView):
    permission_classes = ()
    queryset = SecondModel.objects.all()
    serializer_class = SecondModeSerializer

    def perform_update(self, serializer):
        instance = serializer.save()
        #here i want old value of quantity before updating.

这是我的视图。py:

class FirstModel(models.Model):
    first_model_item = models.ForeignKey(Item)
    quantity =  models.IntegerField()

class SecondModel(models.Model):
    second_model_item = models.ForeignKey(FirstModel)
    quantity = models.IntegerField()

class CreateSecondModelView(generics.CreateAPIView):
    permission_classes = ()
    queryset = SecondModel.objects.all()
    serializer_class = SecondModeSerializer

    def perform_create(self, serializer):
        instance = serializer.save()
        first_model_item = instance.item
        first_model_item.quantity -= instance.quantity
        first_model_item.save()


class UpdateSecondModelView(generics.RetrieveUpdateAPIView):
    permission_classes = ()
    queryset = SecondModel.objects.all()
    serializer_class = SecondModeSerializer

    def perform_update(self, serializer):
        instance = serializer.save()
        #here i want old value of quantity before updating.

---

序列化程序有一个实例属性，在调用save之前不会更新该属性。因此：

old_quantity = serializer.instance.quantity
instance = serializer.save()
new_quantity = instance.quantity