如何基于“最新”相关模型优化排序

Question

所以说我们有两个模型

class Product(models.Model):
    """ A model representing a product in a website. Has new datapoints referencing this as a foreign key daily """
    name = models.CharField(null=False, max_length=1024, default="To be Scraped")
    url = models.URLField(null=False, blank=False, max_length=10000)


class DataPoint(models.Model):
    """ A model representing a datapoint in a Product's timeline. A new one is created for every product daily """
    product = models.ForeignKey(Product, null=False)
    price = models.FloatField(null=False, default=0.0)
    inventory_left = models.BigIntegerField(null=False, default=0)
    inventory_sold = models.BigIntegerField(null=False, default=0)
    date_created = models.DateField(auto_now_add=True)

    def __unicode__(self):
        return "%s - %s" % (self.product.name, self.inventory_sold)

目标是根据附加到产品的最新数据点的inventory_sold 值对产品的查询集进行排序。这是我到目前为止所拥有的：

products = Product.objects.all()
datapoints = DataPoint.objects.filter(product__in=products)

datapoints = list(datapoints.values("product__id", "inventory_sold", "date_created"))
products_d = {}
# Loop over the datapoints values array
for i in datapoints:
    # If a datapoint for the product doesn't exist in the products_d, add the datapoint
    if str(i["product__id"]) not in products_d.keys():
        products_d[str(i["product__id"])] = {"inventory_sold": i["inventory_sold"], "date_created": i["date_created"]}
    # Otherwise, if the current datapoint was created after the existing datapoint, overwrite the datapoint in products_d
    else:
        if products_d[str(i["product__id"])]["date_created"] < i["date_created"]:
            products_d[str(i["product__id"])] = {"inventory_sold": i["inventory_sold"], "date_created": i["date_created"]}
# Sort the products queryset based on the value of inventory_sold in the products_d dictionary
products = sorted(products, key=lambda x: products_d.get(str(x.id), {}).get("inventory_sold", 0), reverse=True)

这可以正常工作，但是对于大量（500,000~）个产品和数据点，它非常慢。有没有更好的方法来做到这一点？

顺便提一下（不重要），由于我找不到任何关于此的信息，因此 DataPoint 模型的 unicode 方法似乎也在进行不必要的 SQL 查询。一旦它们被传递给模板，这是 Django 模型的默认特征吗？

Answer 1

我认为您可以在此处使用子查询来注释最新数据点的值，然后对其进行排序。

根据这些文档中的示例，它将类似于：

from django.db.models import OuterRef, Subquery
newest = DataPoint.objects.filter(product=OuterRef('pk')).order_by('-date_created')
products = Product.objects.annotate(
    newest_inventory_sold=Subquery(newest.values('inventory_sold')[:1])
).order_by('newest_inventory_sold')

对于您的侧点，为了避免在输出 DataPoints 时出现额外的查询，您需要select_related在原始查询中使用：

datapoints = DatePoint.objects.filter(...).select_related('product')

这将执行 JOIN 以便获取产品名称不会导致新的数据库查找。