How to get the old values of django model field while updating without using signals in django rest framework?

Question

I am using django-rest-framework. I have a case in which I need to update the field of other model once instance of first model is created or updated.

here is my models.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()

Here is my views.py:

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.

Daniel Roseman · Accepted Answer · 2018-02-15 13:33:18Z

The serializer has an instance attribute, which is not updated until you call save. So:

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

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