code: def set_context(self, serializer):
          
          # Determine the underlying model field names. These may not be the
          # same as the serializer field names if `source=<>` is set.
          self.field_name = serializer.fields[self.field].source_attrs[0]
          self.date_field_name = serializer.fields[self.date_field].source_attrs[0]
          # Determine the existing instance, if this is an update operation.
          self.instance = getattr(serializer, 'instance', None)
docs: This hook is called by the serializer instance,
          prior to the validation call being made.