[Answered ]-DRF Serializer IntegerField Validation To Make Sure ForeignKey Does Exists

Usually one would use a special field type PrimaryKeyRelatedField.
You don’t even need to manually declare one, as it can be done automatically by the ModelSerializer.

class CartItemCreateSerializer(serializers.ModelSerializer):
    class Meta:
        model = CartItem
        fields = ('id', 'product', 'quantity')

    def save(self, **kwargs):
        cart_id = self.context['cart_id']
        product = self.validated_data['product']
        cart_item, _ = CartItem.objects.get_or_create(
            cart_id=cart_id, product=product)
        cart_item.quantity += self.validated_data['quantity']
        cart_item.save()
        self.instance = cart_item
        return self.instance

https://www.django-rest-framework.org/api-guide/relations/#primarykeyrelatedfield