如何查询模型以获取 ForeignKey 计算字段的聚合

Question

如何查询模型以获取 ForeignKey“计算”（Autofield？）字段的聚合

我有两个模型：

class Job(models.Model):
    name = models.CharField(…)

class LineItem(models.Model):
    job = models.ForeignKey(Job, …)
    metric_1 = models.DecimalField(…)
    metric_2 = models.DecimalField(…)
    metric_3 = models.DecimalField(…)
    # Making this a @property, @cached_property, or the like makes no difference
    def metric_total(self):
        return (self.metric_1 + self.metric_2 * self.metric_3)

在视图中：

class StackoverflowView(ListView, …):
    model = Job

    def get_queryset(self):
        return Job.objects
            .select_related(…)
            .prefetch_related('lineitem_set')
            .filter(…).order_by(…)

    def get_context_data(self, **kwargs):
        context_data = super(StackoverflowView, self).get_context_data(**kwargs)
        context_data['qs_aggregate'] = self.get_queryset() \
            .annotate(
                # Do I need to Annotate the model?
            ).aggregate(
                # This works for any of the fields that have a model field type
                metric1Total=Sum('lineitem__metric_1'),
                metric2Total=Sum('lineitem__metric_2'),
                # This will error : 
                # Unsupported lookup 'metric_total' for AutoField or join on the field not permitted.
                # How do I aggregate the computed model field 'metric_total'?
                metricTotal=Sum('lineitem__metric_total'),
            )
        return context_data

当我尝试聚合计算域时，我收到错误：Unsupported lookup 'metric_total' for AutoField or join on the field not permitted.。如何聚合这些特殊字段？

Answer 1

你也必须计算metric_total

Job.objects.aggregate(
    metric1Total=Sum('lineitem__metric_1'),
    metric2Total=Sum('lineitem__metric_2'),
    metric_total=Sum('lineitem__metric_1') + Sum('lineitem__metric_2')
)

可能的重复：Django @property used in .aggregate()

简而言之，这些annotate() 和aggregate() 方法在数据库级别执行，而metric_total()“方法” 在 Python 级别执行。