django 从具有相同外键的多个表中选择记录

Question

我想在 Django 中执行一个查询，该查询通过外键在多个表中检索相关数据。目前我必须在每个表上运行一个查询，例如（房屋、家具、人）使用门牌号作为过滤器。

在 SQL 中，我可以像这样在一个查询中做到这一点：

SELECT house.number, house.number_of_rooms, furniture.type, people.name
FROM (house INNER JOIN furniture ON house.number = furniture.house_number) 
INNER JOIN people ON house.number = people.house_number
WHERE (((house.number)="21"));

这可以在 Django 中完成吗？

请参阅下面的示例模型：

class House(models.Model):
    number = models.CharField('House Number', max_length=10, blank=True, unique=True, primary_key=True)
    number_of_rooms = models.IntegerField(default=1, null=True)

class Furniture(models.Model):
    house_number = models.ForeignKey(House, on_delete=models.CASCADE, null=True)
    type = models.CharField('Furniture Type', max_length=50)

class People(models.Model):
    house_number = models.ForeignKey(House, on_delete=models.CASCADE, null=True)
    first_name = models.CharField('First Name', max_length=50)

Answer 1

在您的模型中为外键添加 related_name 参数，以便您可以检索与 House() 实例相关的对象。

class Furniture(models.Model):
    house_number = models.ForeignKey(House, related_name='house_furniture', on_delete=models.CASCADE, null=True)
    type = models.CharField('Furniture Type', max_length=50)

class People(models.Model):
    house_number = models.ForeignKey(House, related_name='house_people', on_delete=models.CASCADE, null=True)
    first_name = models.CharField('First Name', max_length=50)

然后使用以下命令运行迁移。

python manage.py makemigrations
python manage.py migrate

然后在同一个应用中创建一个新的 serializers.py 模块。

#import models Furniture, People, house

from rest_framework import serializers

class FurnitureSerializer(serializer.ModelSerializer):

    class Meta:
        model = Furniture
        fields = ['type'] # if you want all the fields of model than user '__all__'.

class PeopleSerializer(serializer.ModelSerializer):
    
    class Meta:
        model = People
        fields = ['first_name'] # if you want all the fields of model than user '__all__'.

class HouseSerializer(serializer.ModelSerializer):

    house_furniture = FurnitureSerializer(many=True)
    house_people = PeopleSerializer(many=True)

    class Meta:
        model = Furniture
        fields = ['number', 'number_of_rooms', 'house_furniture', 'house_people'] 

现在，在您的 views.py 中，您可以简单地查询模型 House 并使用 HouseSerializer() 序列化结果。

#从models.py导入模型 #从 serializers.py 导入序列化程序

从 rest_framework.views 导入 APIView 从 rest_framework.response 导入响应 从 rest_framework 导入状态 从 rest_framework.generics 导入 ListAPIView

类 ListHouseView(ListAPIView)：

serializer_class = HouseSerializer
queryset = House.objects.filter() #here you can apply filters on the fields of house model and user using related_name you can filter on other related models as well.

现在，只需在您应用的 urls.py

中调用 ad this

url_pattern = [
    path('list-house/', ListHouseView.as_view()),
]

确保您的项目的 urls.py 中有一条路径可以访问此应用的 urls.py。

Answer 2

处理此问题的常用 Django 方法是 Queryset.prefetch_related() 并通过 Python 进行迭代（除非您使用 Postgres，它有自己的 ArrayAgg 解决方案）。给定您的模型，它将花费三个查询，但您不必处理非规范化的行结果。

h = House.objects.prefetch_related('furniture_set', 'people_set').get(number='21')
for furniture in house.furniture_set.all():
    print(furniture)
for person in house.people_set.all():
    print(people)

prefetch_related() 缓存结果并在评估查询集后在 Python 中“加入”，因此遍历反向关系不会产生额外的查询，并且您可以随意构建/序列化数据.来自这里的原始 SQL 类似于：

SELECT house.number, house.number_of_rooms FROM house WHERE house.number = '1'
SELECT furniture.id, furniture.house_number_id, furniture.type FROM furniture WHERE furniture.house_number_id IN ('1')
SELECT people.id, people.house_number_id, people.first_name FROM people WHERE people.house_number_id IN ('1')

但 Django 会在幕后完成这项工作，这样您就可以在 Python 中处理模型实例。