Django REST framework: object has no attribute after annotation; Got AttributeError when attempting to get a value for field <field> on serializer

I have a custom user model and subscription model, which contains ForeignKeys to subscriber and user to subscribe to.

class Subscription(models.Model):
    user = models.ForeignKey(
        ApiUser,
        on_delete=models.CASCADE,
        related_name='subscriber'
    )
    subscription = models.ForeignKey(
        ApiUser,
        on_delete=models.CASCADE,
        related_name='subscriptions'
    )

Also i have viewset for subscription and two serializers: for write and for read.

class SubscribeViewSet(mixins.ListModelMixin,
                       mixins.CreateModelMixin,
                       mixins.DestroyModelMixin,
                       viewsets.GenericViewSet):
    queryset = ApiUser.objects.all()
    serializer_class = SubscriptionSerializerForRead
    permission_classes = (permissions.AllowAny,)

    def get_queryset(self):
        queryset = ApiUser.objects.all().annotate(
            is_subscribed=Case(
                When(
                    subscribtions__exact=self.request.user.id,
                    then=Value(True)
                ),
                default=Value(False),
                output_field=BooleanField()
            )
        ).order_by('id')
        return queryset

    def get_serializer_context(self):
        context = super().get_serializer_context()
        context.update({'subscription_id': self.kwargs['pk']})
        return context
    
    def get_serializer_class(self):
        if self.request.method not in permissions.SAFE_METHODS:
            return SubscriptionSerializerForWrite
        return SubscriptionSerializerForRead

    @action(
        methods=['POST'],
        detail=True,
        url_path='subscribe'
    )
    def subscribe(self, request, pk):
        serializer = self.get_serializer(data=request.data)
        serializer.is_valid(raise_exception=True)
        serializer.save()
        return Response(serializer.data, status=status.HTTP_201_CREATED)

class SubscriptionSerializerForRead(serializers.ModelSerializer):
    is_subscribed = serializers.BooleanField()

    class Meta:
        model = ApiUser
        fields = (
            'email',
            'id',
            'username',
            'first_name',
            'last_name',
            'is_subscribed'
        )


class SubscriptionSerializerForWrite(serializers.ModelSerializer):
    user = serializers.StringRelatedField(
        required=False,
        read_only=True,
        default=serializers.CurrentUserDefault()
    )
    subscription = serializers.PrimaryKeyRelatedField(
        read_only=True
    )

    class Meta:
        model = Subscription
        fields = ('user', 'subscription')

    def validate(self, attrs):
        attrs['user'] = self.context['request'].user
        attrs['subscription_id'] = self.context['subscription_id']
        if self.context['request'].user.id == self.context['subscription_id']:
            raise serializers.ValidationError(
                'Cannot subscribe to yourself'
                )
        return attrs
    
    def to_representation(self, instance):
        return SubscriptionSerializerForRead(
            instance=instance.subscription
        ).data

After successful subscription i need to return response with subscribed user data and additional value is_subscribed. I tried to do that through annotate queryset in my viewset, but in response i always get an error:

backend-1   | AttributeError: Got AttributeError when attempting to get a value for field `is_subscribed` on serializer `SubscriptionSerializerForRead`.
backend-1   | The serializer field might be named incorrectly and not match any attribute or key on the `ApiUser` instance.
backend-1   | Original exception text was: 'ApiUser' object has no attribute 'is_subscribed'.

How could i make this work and what's causing that issue? Same annotation method works for other m2m related models in my project, but with user model it fails for some reason

The problem was at to_representation function in SubscriptionSerializerForWrite class which is calling SubscriptionSerializerForRead class but the instance was not annotated when created from the serializer.

The solutions could be either:

• Make the is_subscribed optional. By changing it to SerializerMethodField and write a custom logic like return getattr(obj, 'is_subscribed', False)

• Manually set is_subscribed attribute after created (or in to_representation function):

  def create(self, validated_data):
      instance = super().create(validated_data)
      setattr(instance, 'is_subscribed', True)
      return instance