如何按带注释的字段订购组合查询集？答案

【问题标题】：How to order a combined queryset by annotated field?如何按带注释的字段订购组合查询集？
【发布时间】：2022-12-06 23:51:05
【问题描述】：

我们对象的权重由 2 个字段组成。一个字段是重量单位（千克、吨和磅），另一个字段是重量（数字）。我尝试制作一个查询集，将所有权重注释到一个通用单位字段中。问题是，一旦我 order_by 那个带注释的（单位）字段，它仍然按最大数量而不是最大重量排序。

例如 100kg 小于 50t，但订货系统只是按最大的数排序。

这是 filters.py 中的代码：

class AircraftFilter(FilterSet):
    tail_number = CharFilter(field_name="tail_number", lookup_expr="startswith")
    by_weight = CharFilter(method="filter_by_weight")

    class Meta:
        model = Aircraft
        fields = ("tail_number", "by_weight")

    def filter_by_weight(self, qs: QuerySet, value, *args, **kwargs):
        if value != None:
            qs = (
                qs.filter(max_takeoff_weight_unit=2).annotate(
                    mtw_lb=F("max_takeoff_weight") * 2200
                )
                | qs.filter(max_takeoff_weight_unit=1).annotate(
                    mtw_lb=F("max_takeoff_weight") * 2.2
                )
                | qs.filter(max_takeoff_weight_unit=3).annotate(
                    mtw_lb=F("max_takeoff_weight")
                )
            )
            qs = qs.order_by("mtw_lb")
        return qs

查询：

`qs = (Aircraft.objects.all().filter(max_takeoff_weight_unit=2).annotate(mtw_lb=F("max_takeoff_weight")*2200) | Aircraft.objects.all().filter(max_takeoff_weight_unit=1).annotate(mtw_lb=F("max_takeoff_weight") * 2.2) | Aircraft.objects.all().filter(max_takeoff_weight_unit=3).annotate(mtw_lb=F("max_takeoff_weight"))).order_by("mtw_lb")`

和输出：

`<IsActiveModelQuerySet [ID: 5 | weight: 0.05 - (t) , ID: 4 | weight: 0.20 - (t) , ID: 8 | weight: 2.00 - (t) , ID: 7 | weight: 50.00 - (lbs) , ID: 6 | weight: 100.00 - (kg) ]>`

【问题讨论】：

标签： django filter django-queryset annotate

【解决方案1】：

您可以使用 conditional expression [Django-doc] 进行过滤，因此：

from django.db.models import Case, F, When


class AircraftFilter(FilterSet):
    tail_number = CharFilter(field_name='tail_number', lookup_expr='startswith')
    by_weight = CharFilter(method='filter_by_weight')

    class Meta:
        model = Aircraft
        fields = ('tail_number', 'by_weight')

    def filter_by_weight(self, qs: QuerySet, value, *args, **kwargs):
        if value is not None:
            qs = (
                qs.annotate(
                    mtw_lb=Case(
                        When(
                            max_takeoff_weight_unit=2,
                            then=F('max_takeoff_weight') * 2200,
                        ),
                        When(
                            max_takeoff_weight_unit=1,
                            then=F('max_takeoff_weight') * 2.2,
                        ),
                        When(
                            max_takeoff_weight_unit=3,
                            then=F('max_takeoff_weight'),
                        ),
                    )
                )
            ).order_by('mtw_lb')
        return qs

或更简单：

from django.db.models import Case, F, When


class AircraftFilter(FilterSet):
    tail_number = CharFilter(field_name='tail_number', lookup_expr='startswith')
    by_weight = CharFilter(method='filter_by_weight')

    class Meta:
        model = Aircraft
        fields = ('tail_number', 'by_weight')

    def filter_by_weight(self, qs: QuerySet, value, *args, **kwargs):
        if value is not None:
            qs = (
                qs.annotate(
                    mtw_lb=F('max_takeoff_weight')
                    * Case(
                        When(
                            max_takeoff_weight_unit=2,
                            then=Value(2200),
                        ),
                        When(
                            max_takeoff_weight_unit=1,
                            then=Value(2.2),
                        ),
                        When(
                            max_takeoff_weight_unit=3,
                            then=Value(1),
                        ),
                    )
                )
            ).order_by('mtw_lb')
        return qs

【讨论】：