为什么 Django REST Framework 嵌套序列化器 self.instance 总是返回 None？

Question

I have model,

class Profile(models.Model)
   user = models.OneToOneField(User)

And my serializer classes are,

class UserSerializer(serializers.ModelSerializer):

   def validate(self, attrs):
       print(self.instance) # Always prints None
       print(self.parent.instance.user) # Prints User instance
       return attrs

   class Meta:
       model = User

class ProfileSerializer(serializer.ModelSerializer):
   user = UserSerializer()

   class Meta:
      Model = Profile

And my view class,

class ProfileView(APIView):
   def update(self, request):
      profile = ProfileSerializer(request.user.profile, data=request.data, partial=True)
      if profile.is_valid():
          profile.save()
          return Response(status=HTTP_204_NO_CONTENT)
      return Response({'error': profile.errors}, status=status.HTTP_400_BAD_REQUEST)

Why self.instance inside validate() method in nested serializer always returns none?

Answer 1

When do you see that? On creation?

In that case self.instance is populated inside the save() method with the result of create() method that's called after `validate().

Here you can read

When passing an initial object or queryset to a serializer instance, the object will be made available as .instance. If no initial object is passed then the .instance attribute will be None.

Answer 2

When you are in a child serializer, you can access the instance using the parent field, instead the instance field, something like this:

        if self.parent is not None and self.parent.instance is not None:
            instance = getattr(self.parent.instance, self.field_name)
            check_query = check_query.exclude(pk=user.pk)

Answer 3

Seem that you need to do that validation in the parent

https://www.django-rest-framework.org/api-guide/validators/#updating-nested-serializers

In the case of update operations on nested serializers there's no way of applying this exclusion, because the instance is not available.

Again, you'll probably want to explicitly remove the validator from the serializer class, and write the code the for the validation constraint explicitly, in a .validate() method, or in the view.