django - 如何避免在 Django 中使用反向外键进行如此多的查询

Question

这些是我的模型（简化）：

楷模：

class Tariff(models.Model):
    name = models.CharField(max_length=25, blank=True, null=True)
    service = models.ForeignKey(Service, models.DO_NOTHING, blank=True, null=True)
    cost = models.DecimalField(max_digits=18, decimal_places=3, blank=True, null=True)
    price = models.DecimalField(max_digits=18, decimal_places=3, blank=True, null=True)
    datefrom = models.DateField(blank=True, null=True)
    dateto = models.DateField(blank=True, null=True)

    class Meta:
        managed = True
        db_table = 'Tariff'


class Service(models.Model):
    name = models.CharField(max_length=255, blank=True, null=True)
    city = models.ForeignKey(City, models.DO_NOTHING, blank=True, null=True)
    class Meta:
        managed = True
        db_table = 'Service'

class CalendarValue(models.Model):
    calendar = models.ForeignKey(Calendar, models.DO_NOTHING, blank=True, null=True)
    service = models.ForeignKey(Service, models.DO_NOTHING, blank=True, null=True)
    date = models.DateField(blank=True, null=True)
    value = models.CharField(max_length=50, blank=True, null=True)

    class Meta:
        managed = True
        db_table = 'CalendarValue'

序列化器：

class AvailabilitySerializer(serializers.ModelSerializer):
    sum_cost = serializers.DecimalField(18, 3)
    sum_price = serializers.DecimalField(18, 3)
    service__id = serializers.IntegerField()
    service__name = serializers.CharField()
    calendarvalue = serializers.SerializerMethodField()

    class Meta:
        model = Tariff
        fields = ('name', 'sum_cost', 'sum_price', 'service__id', 'service__name', 'calendarvalue')

    def get_calendarvalue(self, tariff):
        start_date = self.context.get('datefrom')
        end_date = self.context.get('dateto')
        service = Service.objects.get(id=tariff['service__id'])
        return CalendarValueSerializer(CalendarValue.objects.filter(service=service,
                                                                    date__range=[start_date, end_date]), read_only=True,
                                       many=True).data  

这很有效，我得到了所有关税，包括他们的服务和每项服务的所有日历值。
但我注意到它对每个服务进行查询以获取其日历值。
有没有办法在没有这么多查询的情况下获取所有数据？
谢谢。

我的观点

queryset = Tariff.objects.filter(datefrom__lte=start_date, dateto__gte=end_date)
queryset = queryset.filter(service__city=city_code)

# group by
queryset = queryset.values('name', 'service__id', 'service__name')\
    .annotate(sum_cost=Sum('cost'), sum_price=Sum('price'), min_datefrom=Min('datefrom'), max_dateto=Max('dateto'))\
    .order_by('service__id', 'name')

context = {'datefrom': start_date, 'dateto': end_date}

serializer = AvailabilitySerializer(queryset, many=True, context=context)

return serializer.data

我已阅读有关 prefetch_related 的信息，但我不知道如何实现它。
谢谢。

标签： djangodjango-rest-framework