Question

请参考下面的代码

交易模型

class Transaction(models.Model)
    current_product_code = models.CharField(....)
    previous_product_code = models.CharField(....)
    @property
    def status(self):
        c_price = Product.objects.get(code=self.current_product_code).price
        p_price = Product.objects.get(code=self.previous_product_code).price
        if c_price == p_price:
            return "Due"
        elif c_price > p_price:
            return "Upgrade"
        else:
            return "Downgrade"

产品型号

class Product(models.Model):
    code = models.CharField(....)
    price = models.DecimalField(....)

我的问题：我如何获取/过滤具有升级/降级/到期状态的交易。我正在尝试创建一个自定义管理员过滤器，该过滤器根据其状态过滤事务，但是我无法放入.filter（）里面的内容，请检查下面的方法

def queryset(self, request, queryset):
    value = self.value()
    if value == 'Upgrade':
        return queryset.filter(***** HERE *****)
    elif value == 'Downgrade':
        return queryset.filter(***** HERE *****)
    elif value == 'Unknown':
        return queryset.filter(***** HERE *****)
        return queryset

Answer 1

您确实应该在ForeignKey和Product之间使用Transaction（对于current_product_code和previous_product_code两者）。这将使您可以轻松地在查询集中使用这些关系。

我建议的模型结构如下：

class Product(models.Model):
    code = models.CharField(....)
    price = models.DecimalField(....)


class Transaction(models.Model)
    # You have to define related_name for at least one of relations below.
    # Without that, automatically generated ones will clash.
    # Also don't foget to change `on_delete` to suit your needs.
    current_product = models.ForeignKey(Product, related_name="current_transactions", on_delete=models.CASCADE)
    previous_product = models.ForeignKey(Product, related_name="previous_transactions", on_delete=models.CASCADE)

    @property
    def status(self):
        # also, no need to do additional queries here manually. You can improve
        # it further by using `select_related` when querying for transactions.
        c_price = self.current_product.price
        p_price = self.previous_product.price
        if c_price == p_price:
            return "Due"
        elif c_price > p_price:
            return "Upgrade"
        else:
            return "Downgrade"

使用该模型结构，查找特定类型的交易将更加容易：

upgrade_transactions = Transaction.objects.filter(current_product__price__gt=F('previous_product__price'))
downgrade_transactions = Transaction.objects.filter(current_product__price__lt=F('previous_product__price'))
due_transactions = Transaction.objects.filter(current_product__price=F('previous_product__price'))

Answer 2

我认为您可以尝试使用Subquery, OuterRef和.annotate()：

if value == 'Upgrade':
    return queryset.annotate(
        previous_price=Subquery(
            Product.objects.filter(
                code=OuterRef("previous_product_code")
            )[:1]
        ),
        current_price=Subquery(
            Product.objects.filter(
                code=OuterRef("current_product_code")
            )[:1]
        ),
    ).filter(current_price__gt=F("previous_price"))
...

Answer 3

请记住，最后，filter（）操作是一个SQL操作，我们应该注意性能问题。

因此，我的建议是：如果您需要按状态进行过滤，则每次保存交易时都要更新产品模型上的状态。您的应用程序将更快并且代码更简洁。

如何基于其他非相关模型过滤Django模型

3 个答案: