Django REST framework 模型中 Many-to-many 关系的序列化

一、模型

# blogs/models.py
from django.db import models


class TagModel(models.Model):
    tag = models.CharField(max_length=20, unique=True)
    description = models.CharField(max_length=100)

    class Meta:
        db_table = 'blog_tags'


class PostModel(models.Model):
    title = models.CharField(max_length=50)
    update_date = models.DateTimeField(auto_now=True)
    tags = models.ManyToManyField(TagModel)

    class Meta:
        db_table = 'blog_posts'

二、Serializers

# blogs/serializers.py
from rest_framework import serializers
from blogs.models import TagModel, PostModel


class TagSerializer(serializers.ModelSerializer):
    class Meta:
        model = TagModel
        fields = '__all__'


class PostSerializer(serializers.ModelSerializer):
    class Meta:
        model = PostModel
        fields = '__all__'

三、视图

# blogs/views.py
from rest_framework.viewsets import ModelViewSet
from blogs.models import TagModel, PostModel
from blogs.serializers import TagSerializer, PostSerializer


class TagViewSet(ModelViewSet):
    queryset = TagModel.objects.all()
    serializer_class = TagSerializer


class PostViewSet(ModelViewSet):
    queryset = PostModel.objects.all()
    serializer_class = PostSerializer

四、URLs

# blogs/urls.py
from django.urls import include, path
from rest_framework import routers
from blogs.views import TagViewSet, PostViewSet

router = routers.DefaultRouter()
router.register(r'tags', TagViewSet)
router.register(r'posts', PostViewSet)

urlpatterns = [
    path('', include(router.urls))
]

修改项目总体的 URL 配置 urls.py：

from django.contrib import admin
from django.urls import path, include

urlpatterns = [
    path('admin/', admin.site.urls),
    path('blog/', include('blogs.urls'))
]

五、效果

# GET http://127.0.0.1:8002/blog/tags/
[
    {
        "description": "Technique for python programming",
        "id": 1,
        "tag": "Python"
    },
    {
        "description": "Shell programming tricks",
        "id": 2,
        "tag": "Shell"
    },
    {
        "description": "Linux system administration",
        "id": 3,
        "tag": "Linux"
    }
]

# GET http://127.0.0.1:8002/blog/posts/
[
    {
        "id": 1,
        "tags": [
            1,
            3
        ],
        "title": "Python for linux administration",
        "update_date": "2020-08-11T07:01:50.893976Z"
    }
]

六、自定义序列化方式

为了在获取 posts 列表时，每条 post 中的 tags 不仅仅显示 id，还显示对应的标签名称。修改代码如下：

# blogs/serializers.py
from rest_framework import serializers
from blogs.models import TagModel, PostModel


class TagSerializer(serializers.ModelSerializer):
    class Meta:
        model = TagModel
        fields = '__all__'


class TagsReadOnly(serializers.ModelSerializer):
    class Meta:
        model = TagModel
        fields = ['id', 'tag']


class PostSerializer(serializers.ModelSerializer):
    tags = TagsReadOnly(many=True)

    class Meta:
        model = PostModel
        fields = '__all__'

效果：

# GET http://127.0.0.1:8002/blog/posts/
[
    {
        "id": 1,
        "tags": [
            {
                "id": 1,
                "tag": "Python"
            },
            {
                "id": 3,
                "tag": "Linux"
            }
        ],
        "title": "Python for linux administration",
        "update_date": "2020-08-11T07:01:50.893976Z"
    }
]

但此时若使用 POST 或 PUT 方法新增或更新数据，会报出如下错误（即在新增或修改 post 的同时，会尝试创建关联的 tags，但这些 tags 本就已经存在，从而导致冲突）：

{
    "tags": [
        {
            "tag": [
                "tag model with this tag already exists."
            ]
        }
    ]
}

测试 POST 和 PUT 动作时使用的 JSON 数据格式如下：

{
    "tags": [
        {
            "id": 1,
            "tag": "Python"
        }
    ],
    "title": "An interesting post"
}

为了使 posts 接口在接收数据时支持列表类型的 tags（类似 "tags": [1, 2, 3] 这种）且能够成功更新，可以选择覆盖 PostSerializer 的 to_internal_value 和 create 方法：

# blogs/serializers.py
from rest_framework import serializers
from blogs.models import TagModel, PostModel


class TagSerializer(serializers.ModelSerializer):
    class Meta:
        model = TagModel
        fields = '__all__'


class TagsReadOnly(serializers.ModelSerializer):
    class Meta:
        model = TagModel
        fields = ['id', 'tag']


class PostSerializer(serializers.ModelSerializer):
    tags = TagsReadOnly(many=True)

    class Meta:
        model = PostModel
        fields = '__all__'

    def to_internal_value(self, data):
        return data

    def create(self, validated_data):
        tags_data = validated_data.pop('tags')
        post = PostModel.objects.create(**validated_data)
        tags = [TagModel.objects.get(
            pk=id) for id in tags_data]
        post.tags.set(tags)
        post.save()
        return post

其中 to_internal_value 方法用于验证前端传入的数据（上述代码中不做任何验证）；create 方法用于新增或更新后台数据。

此时再向 posts 接口 POST 或 PUT 数据时，就可以使用如下格式：

{
    "tags": [1, 2, 3],
    "title": "An interesting post"
}