我有一个视图，它需要来自第三、第四级关系中许多相关对象的数据。
下面是视图：

class OrderViewSet(ModelViewSet):
    queryset = Order.objects.select_related().prefetch_related().order_by('-id')

现在，在序列化器中，我必须获取此订单的相关项目的库存，然后对其执行某些计算。

class OrderListSerializer(ModelSerializer):
    shortages = SerializerMethodField()
    def get_shortages(self, obj):
        item = obj.order_line.product.item
        stock =  (
                order_line.product.item.item_instances.filter(stock_quantity__gt=0)
                .aggregate(stock=Coalesce(Sum("stock_quantity"), 0, output_field=FloatField()))
                .get("stock", 0)
            )
        required_stock = self.calculate_required_stock()
        if stock < required_stock: .....

在上面的代码中，order_line.product.item.item_instances行表示4个模型，如果我用('order_line__product__item')和('order_line__product__item__item_instances')填充查询集的prefetch_related，发送到数据库的查询仍然是以千为单位的。
我怎样才能改善这种情况。

更新新增示例机型

class Order(models.Model):
    order_line = models.ForeignKey(
        "OrderLine",
        on_delete=models.CASCADE,
        related_name="orders",
    )
    type = models.CharField(choices=TYPE_CHOICES, max_length=13, blank=True)
    start_date = models.DateField(blank=True, null=True)
    ....................

class OrderLine(models.Model):
    sales_order = models.ForeignKey(
        "SalesOrder",
        on_delete=models.CASCADE,
        related_name="sales_order_lines",
    )
    product = models.ForeignKey("Product", on_delete=models.PROTECT)
    quantities = models.JSONField(null=True, blank=True)
    ..............................

class Product(models.Model):
    design = models.ForeignKey(
        Design, on_delete=models.PROTECT, related_name="design_products"
    )
    item = models.ForeignKey(
        Item, on_delete=models.PROTECT, related_name="item_products"
    )
    ...............................

class Item(models.Model):
    shrinkage = models.FloatField(default=0.0)
    ..............................

class ItemInstance(models.Model):
    item = models.ForeignKey(
        Item, on_delete=models.PROTECT, related_name="item_instances"
    )
    stock_quantity = models.FloatField(
        default=0.0, blank=True, validators=[MinValueValidator(0)]
    )