Django 注释查询集，其中包含某些字段的最新值

问题描述

我有一个模型来存储一些营销表现：

class DailyPerformance(models.Model):

    class Meta:
        unique_together = ('route_hash_value','datetime_tz')
        ordering = ('-datetime_tz',)

    route_hash_value = models.CharField(max_length=255,db_index=True,editable=False)
    datetime_tz = models.DateTimeField(db_index=True,editable=False)
    network = models.ForeignKey('administration.Network',on_delete=models.PROTECT,null=True)
    account = models.ForeignKey('administration.Account',on_delete=models.SET_NULL,null=True)

    # Properties
    budget = models.FloatField(null=True,db_index=True)
    bid = models.FloatField(null=True,db_index=True)
    status = models.CharField(max_length=255,null=True,db_index=True)

    # METRICS
    cost_amount = models.FloatField(null=True,db_index=True)
    impressions = models.IntegerField(null=True,db_index=True)
    clicks_in = models.IntegerField(null=True,db_index=True)
    clicks_out = models.IntegerField(null=True,db_index=True)
    revenue_amount_net = models.FloatField(null=True,db_index=True)
    _profit = models.FloatField(null=True,db_index=True)
    _roi = models.FloatField(null=True,db_index=True)
    _ctr = models.FloatField(null=True,db_index=True)

我需要为 drf ModelViewset 生成一个查询集，它聚合数字指标并获取字段的最后一个值：出价、预算和状态。

像这样：

class DailyPerformanceHViewSet(viewsets.ReadOnlyModelViewSet):

    def get_queryset(self):
        queryset = DailyPerformance.objects.values(
            'route_hash_value','datetime_tz','network','account',).annotate(
            cost_amount=Round(Sum('cost_amount'),3),revenue_amount_net=Round(Sum('revenue_amount_net'),impressions=Sum('cost_impressions',output_field=FloatField()),clicks_in=Sum('cost_clicks_in',clicks_out=Sum('cost_clicks_out',_profit=Round(Sum('_profit'),_roi=Round(F('_profit') / F('cost_amount'),_ctr=Round(F('clicks_out') / F('clicks_in'),### 
            budget=...,# <== LATEST BUDGET ORDERED BY -datetime_tz 
            bid=...,# <== LATEST BID ORDERED BY -datetime_tz 
            status=...,# <== LATEST STATUS ORDERED BY -datetime_tz 
        )

        return queryset

但我真的不知道该怎么做

解决方法

您好，您可能需要执行子查询以将最新预算等注释到您的查询集中。

# this will create a query to select DailyPerformance objects and order by date.
last_budget_entry = DailyPerformance.objects.filter(id=OuterRef('pk')).order_by('-datetime_tz)

# annotate your queryset with the query first item in the query(since its the newest). 
queryset.annotate(latest_budget=Subquery(last_budget_entry.values('id')[:1]))

这将创建一个子查询，它将评估“last_budget_entry”查询中第一个元素的 id。 （该值将由 Outeref 提供）并将 id 的值注释到查询集。

#to annotate the value of the budget field 
queryset.annotate(latest_budget=Subquery(last_budget_entry.values('budget')[:1]))

您可以使用outerref对查询进行排序以选择要在新查询集上注释的值 在此处阅读有关查询表达式和子查询的信息 https://docs.djangoproject.com/en/3.1/ref/models/expressions/#subquery-expressions