Django - 预取查询不适用于相关映射表

Question

我正在尝试在遍历多个映射表时预取和选择相关的一些外键模型。但是我仍然遇到 500 多个查询，我设法将其从 1000 降低，但看起来子网字段并没有被获取。

我尝试了以下方法：

circuits = SiteCircuits.objects.all() \
                            .exclude(circuit__decommissioned=True) \
                            .select_related('site') \
                            .select_related('circuit') \
                            .prefetch_related(
                                Prefetch(
                                    'circuit__devicecircuitsubnets_set',
                                    queryset=DeviceCircuitSubnets.objects.all() \
                                    .select_related('subnet')
                                    )
                             ) \
                            .prefetch_related('circuit__circuitnotes_set') \
                            .prefetch_related('circuit__circuit_type') \
                            .prefetch_related('circuit__circuitfile') \
                            .prefetch_related('circuit__service_provider') \
                            .prefetch_related('circuit__circuit_type')

还有这种方式：

circuits = SiteCircuits.objects.all() \
                            .exclude(circuit__decommissioned=True) \
                            .select_related('site') \
                            .select_related('circuit') \
                            .prefetch_related('circuit__devicecircuitsubnets_set') \
                            .prefetch_related('circuit__devicecircuitsubnets_set__subnet') \
                            .prefetch_related('circuit__circuitnotes_set') \
                            .prefetch_related('circuit__circuit_type') \
                            .prefetch_related('circuit__circuitfile') \
                            .prefetch_related('circuit__service_provider') \
                            .prefetch_related('circuit__circuit_type')

但我有 500 多个以下查询，我认为可以通过在子网上选择相关来获得：

SELECT "config_devicecircuitsubnets"."id", "config_devicecircuitsubnets"."device_id", "config_devicecircuitsubnets"."circuit_id", "config_devicecircuitsubnets"."subnet_id", "circuits_circuit"."id", "circuits_circuit"."name", "circuits_circuit"."order_no", "circuits_circuit"."ref_no", "circuits_circuit"."expected_install_date", "circuits_circuit"."install_date", "circuits_circuit"."circuit_type_id", "circuits_circuit"."preference", "circuits_circuit"."service_provider_id", "circuits_circuit"."username", "circuits_circuit"."password", "circuits_circuit"."tel_no", "circuits_circuit"."cost_per_month", "circuits_circuit"."contract_length", "circuits_circuit"."speed_down", "circuits_circuit"."speed_up", "circuits_circuit"."rssi", "circuits_circuit"."bearer", "circuits_circuit"."decommissioned", "config_subnet"."id", "config_subnet"."subnet", "config_subnet"."mask", "config_subnet"."subnet_type_id" FROM "config_devicecircuitsubnets" INNER JOIN "circuits_circuit" ON ("config_devicecircuitsubnets"."circuit_id" = "circuits_circuit"."id") INNER JOIN "config_subnet" ON ("config_devicecircuitsubnets"."subnet_id" = "config_subnet"."id") WHERE "config_devicecircuitsubnets"."circuit_id" = '1' ORDER BY "config_devicecircuitsubnets"."id" ASC LIMIT 1
  Duplicated 526 times.

线路错误：

{{ item.circuit.devicecircuitsubnets_set.first.subnet }}{{ item.circuit.devicecircuitsubnets_set.first.mask }}

高级模型：

-Site
-Circuit
-Device 
-Subnet
--SiteCircuits (mapping table)
--DeviceCircuitSubnets (mapping table)

编辑：

我尝试了一个没有抛出任何错误的子查询，但我不确定它是否正确或如何访问该字段？

子查询示例：

circuit_subnet = DeviceCircuitSubnets.objects.filter(circuit=OuterRef('pk'))
circuits = SiteCircuits.objects.all() \
                        .exclude(circuit__decommissioned=True) \
                        .select_related('site') \
                        .select_related('circuit') \
                        .prefetch_related('circuit__devicecircuitsubnets_set') \
                        .prefetch_related('circuit__devicecircuitsubnets_set__subnet') \
                        .prefetch_related('circuit__circuitnotes_set') \
                        .prefetch_related('circuit__circuit_type') \
                        .prefetch_related('circuit__circuitfile') \
                        .prefetch_related('circuit__service_provider') \
                        .prefetch_related('circuit__circuit_type') \
                        .annotate(circuit__devicecircuitsubnets_set=Subquery(circuit_subnet.values('subnet')[:1]))

Answer 1

使用.first 可能会导致额外的查询。尝试改用.0。

{{ item.circuit.devicecircuitsubnets_set.first.subnet }}{{ item.circuit.devicecircuitsubnets_set.first.mask }}

如果您只需要预取第一个相关项目，那么您也许可以使用subquery。