如何在djongo中查询关联外键的所有邮件组及用户信息

问题回顾 #

我使用djongo作为MongoDB的数据库连接器，创建了MailGroup和User两个模型，每个User关联一个MailGroup，每个MailGroup可包含多个User。期望获取如下格式的嵌套数据：

mailGroups = [
    {
        "id": 1,
        "name": "group - 1",
        "users": [
            {
                "name": "x1",
                "email": "y1"
            },
            {
                "name": "x2",
                "email": "y2"
            }
        ]
    }
]

但当前执行以下代码仅能获取邮件组信息，无法拿到关联的用户：

mailGroups = MailGroup.objects.all()
serializer = MailGroupSerializer(mailGroups, many = True)

模型代码：

class MailGroup(models.Model):
    id = models.AutoField(primary_key=True)
    name = models.CharField(max_length=255)

class User(models.Model):
    id = models.AutoField(primary_key=True)
    email = models.CharField(max_length=255)
    name = models.CharField(max_length=255)
    mailgroup = models.ForeignKey(MailGroup, on_delete=models.CASCADE)

序列化器代码：

class MailGroupSerializer(serializers.ModelSerializer):
    class Meta:
        model = MailGroup
        fields = ["id", "name"]

class RecipientSerializer(serializers.ModelSerializer):
    class Meta:
        model = Recipient
        fields = ["id", "email", "name", "mailGroup"]

解决步骤 #

1. 优化模型反向关联（可选但推荐） #

在User模型的ForeignKey字段中指定related_name，让反向关联名称更直观：

class User(models.Model):
    id = models.AutoField(primary_key=True)
    email = models.CharField(max_length=255)
    name = models.CharField(max_length=255)
    # 指定反向关联名为users，方便后续调用
    mailgroup = models.ForeignKey(MailGroup, on_delete=models.CASCADE, related_name="users")

若不指定related_name，默认反向关联名称为user_set，后续步骤需对应替换。

2. 修改序列化器实现嵌套输出 #

创建仅包含用户姓名和邮箱的序列化器，再在MailGroupSerializer中嵌套该序列化器，关联到反向字段：

# 定义简化的用户序列化器，只返回需要的字段
class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ["name", "email"]

# 修改邮件组序列化器，嵌套用户序列化器
class MailGroupSerializer(serializers.ModelSerializer):
    # 关联反向字段users（未指定related_name则用user_set）
    users = UserSerializer(many=True, read_only=True)
    
    class Meta:
        model = MailGroup
        # 包含id、name和嵌套的users字段
        fields = ["id", "name", "users"]

注：原代码中RecipientSerializer对应模型应为User，此处统一修正为UserSerializer。

3. 优化查询避免N+1问题 #

使用prefetch_related预加载关联用户数据（djongo对MongoDB支持该操作）：

# 预加载每个邮件组对应的用户
mailGroups = MailGroup.objects.all().prefetch_related("users")
serializer = MailGroupSerializer(mailGroups, many=True)
# serializer.data即为所需的嵌套格式数据

完整代码示例 #

模型文件

from django.db import models

class MailGroup(models.Model):
    id = models.AutoField(primary_key=True)
    name = models.CharField(max_length=255)

class User(models.Model):
    id = models.AutoField(primary_key=True)
    email = models.CharField(max_length=255)
    name = models.CharField(max_length=255)
    mailgroup = models.ForeignKey(MailGroup, on_delete=models.CASCADE, related_name="users")

序列化器文件

from rest_framework import serializers
from .models import MailGroup, User

class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ["name", "email"]

class MailGroupSerializer(serializers.ModelSerializer):
    users = UserSerializer(many=True, read_only=True)
    
    class Meta:
        model = MailGroup
        fields = ["id", "name", "users"]

查询代码

from .models import MailGroup
from .serializers import MailGroupSerializer

def get_mail_groups():
    mailGroups = MailGroup.objects.all().prefetch_related("users")
    serializer = MailGroupSerializer(mailGroups, many=True)
    return serializer.data

内容的提问来源于stack exchange，提问作者user22168223