class AddCartItemSerializer(serializers.ModelSerializer):
    product_id = serializers.IntegerField()

    def validate_product_id(self, value):
        if not Product.objects.filter(pk=value).exists():
            raise serializers.ValidationError('No product with the given ID was found.')
        return value

    def save(self, **kwargs):
        cart_id = self.context['cart_id']
        product_id = self.validated_data['product_id']
        quantity = self.validated_data['quantity']

        try: 
            cart_item = CartItem.objects.get(cart_id=cart_id, product_id=product_id)
            cart_item.quantity += quantity
            cart_item.save()
            self.instance = cart_item
        except CartItem.DoesNotExist:
            self.instance = CartItem.objects.create(cart_id=cart_id, **self.validated_data)
        
        return self.instance

    class Meta:
        model = CartItem
        fields = ['id', 'product_id', 'quantity']


class UpdateCartItemSerializer(serializers.ModelSerializer):
    class Meta:
        model = CartItem
        fields = ['quantity']


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class CartItemViewSet(ModelViewSet):
    http_method_names = ['get', 'post', 'patch', 'delete']
   
    def get_serializer_class(self):
        if self.request.method == 'POST':
            return AddCartItemSerializer
        elif self.request.method == 'PATCH':
            return UpdateCartItemSerializer
        return CartItemSerializer

    def get_serializer_context(self):
        return {'cart_id': self.kwargs['cart_pk']}

    def get_queryset(self):
        return CartItem.objects \
                .filter(cart_id=self.kwargs['cart_pk']) \
                .select_related('product')