django开发手册DRF提升Django查询效率，你需要知道的重要操作

Posted 2023-04-02 帅气的黑桃J

前言

在Django ORM中，使用prefetch_related可以避免N+1查询问题，提高查询性能。在实际的开发中，我们通常需要在序列化时嵌套使用prefetch_related，将嵌套数据一起序列化出来。

快速上手

以下是一个案例，一个项目Project包含多个Bim模型BimFaceModel，需要在查询Project时同时查询其对应的BimFaceModel信息。

普通的查询代码如下：

class ProjectViewSet(viewsets.ModelViewSet):
    queryset = models.Project.objects.all()
    serializer_class = ProjectSerializer

    def list(self, request, *args, **kwargs):
        queryset = self.filter_queryset(self.get_queryset())

        # 这里是关键，每个查询都会再执行一次查询
        for project in queryset:
            models.BimFaceModel.objects.filter(project_id=project.project_id)

        serializer = self.get_serializer(queryset, many=True)
        return ResponseResult(data=serializer.data).to_response()

可以看到，如果不加处理，每次循环都需要再次查询对应的BimFaceModel信息，会产生N+1查询问题，影响性能。

使用prefetch_related进行优化：

class ProjectViewSet(viewsets.ModelViewSet):
    queryset = models.Project.objects.all()
    serializer_class = ProjectSerializer

    def list(self, request, *args, **kwargs):
        queryset = self.get_queryset().prefetch_related('bimfacemodel_set')
        serializer = self.get_serializer(queryset, many=True)
        return ResponseResult(data=serializer.data).to_response()

可以看到使用prefetch_related方法来提前加载Project对象的bimfacemodel_set属性。然后在序列化时，可以将每个Project对象的bimfacemodel_set一起序列化出来。

但是，在Project模型中，没有bimfacemodel_set这个属性，只是对于每个BimFaceModel都有对应的project_id作为外键。为了嵌套序列化BimFaceModel，需要在ProjectSerializer中使用BimFaceModelSerializer进行嵌套序列化，具体方法请见以下代码：

class BimFaceModelSerializer(serializers.ModelSerializer):
    class Meta:
        model = BimFaceModel
        fields = ('model_id', 'model_name', 'field_id', 'x', 'y', 'z')


class ProjectSerializer(serializers.ModelSerializer):
    bimfacemodel_set = BimFaceModelSerializer(many=True, read_only=True)

    class Meta:
        model = Project
        fields = ('project_id', 'project_name', 'project_description', 'project_img', 'project_address', 'project_price', 'project_company',
                  'project_area', 'project_cycle', 'project_duty_com', 'project_design_com', 'project_check_com', 'project_build_com',
                  'creator_id', 'create_time', 'update_id', 'update_time', 'bimfacemodel_set')

通过上述方式，可以把每个Project对象对应的所有BimFaceModel序列化出来并放到bimfacemodel_set字段中。这样，就避免了N+1查询问题，在序列化数据时，也能够嵌套序列化。

django drf 改变retrive的pk查询字段

lookup_filed可以改变retrive查询时默认以pk查询的逻辑

 

from django.shortcuts import render

from rest_framework import mixins,viewsets
from .serializers import UserFavSerializer
from .models import UserFav
from rest_framework.permissions import IsAuthenticated
# Create your views here.
from rest_framework import permissions
class IsOwnerOrReadOnly(permissions.BasePermission):
    """
    Object-level permission to only allow owners of an object to edit it.
    Assumes the model instance has an `owner` attribute.
    """

    def has_object_permission(self, request, view, obj):
        # Read permissions are allowed to any request,
        # so we‘ll always allow GET, HEAD or OPTIONS requests.
        if request.method in permissions.SAFE_METHODS:
            return True

        # Instance must have an attribute named `owner`.
        return obj.user == request.user

class UserFavSetview(mixins.CreateModelMixin,mixins.ListModelMixin,
                     mixins.RetrieveModelMixin,
                     mixins.DestroyModelMixin,viewsets.GenericViewSet):
    permission_classes = (IsAuthenticated,IsOwnerOrReadOnly) #需登陆和需要是拥有者
    serializer_class = UserFavSerializer
    lookup_field = ‘goods_id‘
    # queryset = UserFav.objects.all()
    def get_queryset(self):
        return UserFav.objects.filter(user=self.request.user)