DRF-Serialization

按照上一章内容的架构图自下向上，本章介绍Serialization。关于Model层，在原先的Django基础上未做修改，故不做介绍。若想了解可参看官方文档或可参看我之前写的Django模型简介

Introduction

首先思考一下，若不使用DRF框架，我们生成一个API接口应该会怎么做呢？比如传入一个用户名，需要返回用户的相关信息。大概会这么实现：

def get_user_info(request):
    if request.method == 'GET':
        username = request.GET.get('username', None)
        try:
            user = User.objects.get(username=username)
        except User.DoesNotExist:
            user = None

        result = {}
        if not user:
            result = {
                'name': user.username,
                'email': user.email,
                'is_superuser': user.is_superuser,
            }
        return JsonResponse(result)

我们需要做的，首先是解析从客户端传来的参数，然后按照要求获取相关信息，最后组成需要返回的数据。

如果客户端传入的参数，以及需要返回的数据特别多，还按照上述方式进行处理的话，一个视图函数将会变得十分臃肿。而且一般我们的api接口大部分是对数据库表文件进行增删查改，每个功能都只用这么一个视图函数来实现的话，会出现许多重复性代码。

DRF在models层增加一层Serialization，主要用于解析客户端传入的json参数将其转化为django可用的数据。以及将models的数据序列化为json数据，用于返回给客户端。

接下来，让我们用官方文档中的例子，具体看看Serialization

Creating a model to work with

因为Serialization一个重要的功能就是序列化models数据，甚至Serialization可以直接与model绑定。所以首先，让我们来创建一个model： snippets/models.py

from django.db import models
from pygments.lexers import get_all_lexers
from pygments.styles import get_all_styles

LEXERS = [item for item in get_all_lexers() if item[1]]
LANGUAGE_CHOICES = sorted([(item[1][0], item[0]) for item in LEXERS])
STYLE_CHOICES = sorted((item, item) for item in get_all_styles())


class Snippet(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    title = models.CharField(max_length=100, blank=True, default='')
    code = models.TextField()
    linenos = models.BooleanField(default=False)
    language = models.CharField(choices=LANGUAGE_CHOICES, default='python', max_length=100)
    style = models.CharField(choices=STYLE_CHOICES, default='friendly', max_length=100)

    class Meta:
        ordering = ('created',)

Serialization

Creating a Serializer class

snippets/serializers.py

from rest_framework import serializers
from snippets.models import Snippet, LANGUAGE_CHOICES, STYLE_CHOICES

class SnippetSerializer(serializers.Serializer):
    id = serializers.IntegerField(read_only=True)
    title = serializers.CharField(required=False, allow_blank=True, max_length=100)
    code = serializers.CharField(style={'base_template': 'textarea.html'})
    linenos = serializers.BooleanField(required=False)
    language = serializers.ChoiceField(choices=LANGUAGE_CHOICES, default='python')
    style = serializers.ChoiceField(choices=STYLE_CHOICES, default='friendly')

    def create(self, validated_data):
        """
        Create and return a new `Snippet` instance, given the validated data.
        """
        return Snippet.objects.create(**validated_data)

    def update(self, instance, validated_data):
        """
        Update and return an existing `Snippet` instance, given the validated data.
        """
        instance.title = validated_data.get('title', instance.title)
        instance.code = validated_data.get('code', instance.code)
        instance.linenos = validated_data.get('linenos', instance.linenos)
        instance.language = validated_data.get('language', instance.language)
        instance.style = validated_data.get('style', instance.style)
        instance.save()
        return instance

可以看到serializer类的第一部分定义了我们需要序列化或反序列化的字段，非常类似于Django 的Form类，并且每个字段中都包含了相似的认证标志，包括required, max_length 和 default。

第二部分定义两个类函数：create和update，分别对应着Snippet model实例的创建和更新。当调用serializer.save()时，会调用这两个函数。让我们来看看DRF源码中serializers.py文件中对应的代码就会明白：

def save(self, **kwargs):
    ……
    if self.instance is not None:
        self.instance = self.update(self.instance, validated_data)
        assert self.instance is not None, (
            '`update()` did not return an object instance.'
        )
    else:
        self.instance = self.create(validated_data)
        assert self.instance is not None, (
                '`create()` did not return an object instance.'
        )

    return self.instance

可以看到，当我们调用serializer.save()时，若serializer中对应的model实例不存在时创建实例，若存在即更新实例。

接下来，让我们看看我们定义的SnippetSerializer类的功能

序列化

创建实例

from snippets.models import Snippet
from snippets.serializers import SnippetSerializer
from rest_framework.renderers import JSONRenderer
from rest_framework.parsers import JSONParser

snippet = Snippet(code='foo = "bar"\n')
snippet.save()

snippet = Snippet(code='print "hello, world"\n')
snippet.save()

序列化

serializer = SnippetSerializer(snippet)
serializer.data
# {'id': 2, 'title': u'', 'code': u'print "hello, world"\n', 'linenos': False, 'language': u'python', 'style': u'friendly'}

serializer_list = SnippetSerializer(Snippet.objects.all(), many=True)
serializer_list.data
# [OrderedDict([('id', 1), ('title', u''), ('code', u'foo = "bar"\n'), ('linenos', False), ('language', 'python'), ('style', 'friendly')]), OrderedDict([('id', 2), ('title', u''), ('code', u'print "hello, world"\n'), ('linenos', False), ('language', 'python'), ('style', 'friendly')]), OrderedDict([('id', 3), ('title', u''), ('code', u'print "hello, world"'), ('linenos', False), ('language', 'python'), ('style', 'friendly')])]

可以看到serializer类序列化后的字符串格式为Unicode格式，可用JSONRenderer()将其渲染成普通字符串。

content = JSONRenderer().render(serializer.data)
content
# '{"id": 2, "title": "", "code": "print \\"hello, world\\"\\n", "linenos": false, "language": "python", "style": "friendly"}'

反序列化

格式转换

import io

stream = io.BytesIO(content)
data = JSONParser().parse(stream)

反序列化

serializer = SnippetSerializer(data=data)
serializer.is_valid()
# True
serializer.validated_data
# OrderedDict([('title', ''), ('code', 'print "hello, world"\n'), ('linenos', False), ('language', 'python'), ('style', 'friendly')])
serializer.save()
# <Snippet: Snippet object>

注意：当对数据进行反序列化时，必须对数据进行可靠性检验，即调用serializer.is_valid()。还可以重载此函数，加入自己的检验逻辑。

还是继续看看源码，首先是serializer类的构造函数，可追溯到其父类BaseSerializer类的构造函数：

class BaseSerializer(Field):
    '''
    In particular, if a `data=` argument is passed then:

    .is_valid() - Available.
    .initial_data - Available.
    .validated_data - Only available after calling `is_valid()`
    .errors - Only available after calling `is_valid()`
    .data - Only available after calling `is_valid()`

    If a `data=` argument is not passed then:

    .is_valid() - Not available.
    .initial_data - Not available.
    .validated_data - Not available.
    .errors - Not available.
    .data - Available.
    '''

    def __init__(self, instance=None, data=empty, **kwargs):
        self.instance = instance
        if data is not empty:
            self.initial_data = data
        self.partial = kwargs.pop('partial', False)
        self._context = kwargs.pop('context', {})
        kwargs.pop('many', None)
        super(BaseSerializer, self).__init__(**kwargs)
    ……

可以看到当传入model实例，对实例进行序列化；当传入data时，对data进行反序列化。

并且当传入data与否，函数应按照源码注释中所言的限制来使用。

ModelSerializers

SnippetSerializer类与Snippet类仍然有许多重复的字段的信息，如果能够再简洁一些就更好了。于是DRF 封装了ModelSerializers类，直接与Model相绑定。于是SnippetSerializer类变成了：

class SnippetSerializer(serializers.ModelSerializer):
    class Meta:
        model = Snippet
        fields = ('id', 'title', 'code', 'linenos', 'language', 'style')

使用命令python manage.py shell，尝试如下操作：

from snippets.serializers import SnippetSerializer
serializer = SnippetSerializer()
print(repr(serializer))
# SnippetSerializer():
#    id = IntegerField(label='ID', read_only=True)
#    title = CharField(allow_blank=True, max_length=100, required=False)
#    code = CharField(style={'base_template': 'textarea.html'})
#    linenos = BooleanField(required=False)
#    language = ChoiceField(choices=[('Clipper', 'FoxPro'), ('Cucumber', 'Gherkin'), ('RobotFramework', 'RobotFramework'), ('abap', 'ABAP'), ('ada', 'Ada')...
#    style = ChoiceField(choices=[('autumn', 'autumn'), ('borland', 'borland'), ('bw', 'bw'), ('colorful', 'colorful')...

ModelSerializers类只是对原来的Serializers类进行了封装，内部实现了字段列表及create、update等函数的实现。从而使得，当继承ModelSerializers类时，代码变得更加简洁。但实际上两者完成的功能一致。

Writing regular Django views using our Serializer

如上， Serializers基本已介绍完毕。接下来用我们实现的SnippetSerializer，完成部分API接口。

snippets/views.py:

from django.http import HttpResponse, JsonResponse
from django.views.decorators.csrf import csrf_exempt
from rest_framework.renderers import JSONRenderer
from rest_framework.parsers import JSONParser
from snippets.models import Snippet
from snippets.serializers import SnippetSerializer

@csrf_exempt
def snippet_list(request):
    """
    List all code snippets, or create a new snippet.
    """
    if request.method == 'GET':
        snippets = Snippet.objects.all()
        serializer = SnippetSerializer(snippets, many=True)
        return JsonResponse(serializer.data, safe=False)

    elif request.method == 'POST':
        data = JSONParser().parse(request)
        serializer = SnippetSerializer(data=data)
        if serializer.is_valid():
            serializer.save()
            return JsonResponse(serializer.data, status=201)
        return JsonResponse(serializer.errors, status=400)

@csrf_exempt
def snippet_detail(request, pk):
    """
    Retrieve, update or delete a code snippet.
    """
    try:
        snippet = Snippet.objects.get(pk=pk)
    except Snippet.DoesNotExist:
        return HttpResponse(status=404)

    if request.method == 'GET':
        serializer = SnippetSerializer(snippet)
        return JsonResponse(serializer.data)

    elif request.method == 'PUT':
        data = JSONParser().parse(request)
        serializer = SnippetSerializer(snippet, data=data)
        if serializer.is_valid():
            serializer.save()
            return JsonResponse(serializer.data)
        return JsonResponse(serializer.errors, status=400)

    elif request.method == 'DELETE':
        snippet.delete()
        return HttpResponse(status=204)

snippets/urls.py:

from django.urls import path
from snippets import views

urlpatterns = [
    path('snippets/', views.snippet_list),
    path('snippets/<int:pk>/', views.snippet_detail),
]