默认情况下，`.create（）`方法不支持可写嵌套字段。

Question

对于DRF中与中间模型的多对多关系的序列化，我有一个很大的问题：如果请求方法是get everything，那么一切都可以正常工作。但是，只要我尝试向API发布或放置数据，就会出现以下错误：

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/djangorestframework-3.5.3-py2.7.egg/rest_framework/views.py", line 477, in dispatch
    response = self.handle_exception(exc)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/djangorestframework-3.5.3-py2.7.egg/rest_framework/views.py", line 437, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/djangorestframework-3.5.3-py2.7.egg/rest_framework/views.py", line 474, in dispatch
    response = handler(request, *args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/djangorestframework-3.5.3-py2.7.egg/rest_framework/generics.py", line 243, in post
    return self.create(request, *args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/djangorestframework-3.5.3-py2.7.egg/rest_framework/mixins.py", line 21, in create
    self.perform_create(serializer)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/djangorestframework-3.5.3-py2.7.egg/rest_framework/mixins.py", line 26, in perform_create
    serializer.save()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/djangorestframework-3.5.3-py2.7.egg/rest_framework/serializers.py", line 214, in save
    self.instance = self.create(validated_data)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/djangorestframework-3.5.3-py2.7.egg/rest_framework/serializers.py", line 888, in create
    raise_errors_on_nested_writes('create', self, validated_data)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/djangorestframework-3.5.3-py2.7.egg/rest_framework/serializers.py", line 780, in raise_errors_on_nested_writes
    class_name=serializer.__class__.__name__
AssertionError: The `.create()` method does not support writable nested fields by default.
Write an explicit `.create()` method for serializer `manager.serializers.EquipmentSerializer`, or set `read_only=True` on nested serializer fields.

我不太清楚如何编写正确的创建和更新函数，我也不太明白，文档中是如何解释的。

代码：

视图.py：

from django.shortcuts import render 
from django.contrib.auth.models import User, Group
from manager.serializers import *
from rest_framework import generics 
from rest_framework import viewsets 
from rest_framework.decorators import api_view 
from rest_framework.response import Response 
from rest_framework.views import APIView  
from django.forms.models import model_to_dict



class OrderSetDetails(generics.RetrieveUpdateDestroyAPIView): 
    queryset = Order.objects.all() 
    serializer_class = OrderSerializer 

class OrderSetList(generics.ListCreateAPIView): 
    queryset = Order.objects.all() 
    serializer_class = OrderSerializer 

class EquipmentSetDetails(generics.RetrieveUpdateDestroyAPIView): 
    queryset = Equipment.objects.all() 
    serializer_class = EquipmentSerializer 

class EquipmentSetList(generics.ListCreateAPIView): 
    queryset = Equipment.objects.all() 
    serializer_class = EquipmentSerializer


class UserViewSet(viewsets.ModelViewSet):

    queryset = User.objects.all().order_by('-date_joined')
    serializer_class = UserSerializer


class GroupViewSet(viewsets.ModelViewSet):

    queryset = Group.objects.all()
    serializer_class = GroupSerializer 

class ClientList(generics.ListCreateAPIView): 
    queryset = client.objects.all() 
    serializer_class = ClientSerializer  

序列化程序.py

from rest_framework import serializers  
from django.contrib.auth.models import User, Group
from storage.models import * 


class AssignmentSerializer(serializers.HyperlinkedModelSerializer): 
    id = serializers.ReadOnlyField(source = 'Order.id') 
    name = serializers.ReadOnlyField(source = 'Order.name') 

    class Meta:
        model = Assignment 
        fields = ('id', 'name', 'quantity') 


class EquipmentSerializer(serializers.ModelSerializer): 
    event = AssignmentSerializer(source= 'assignment_set', many = True)
    class Meta: 
        model = Equipment 
        fields = '__all__' 


class ClientSerializer(serializers.ModelSerializer): 

    class Meta: 
        model = client 
        fields = '__all__' 

class UserSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = User
        fields = ('url', 'username', 'email', 'groups')


class GroupSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Group
        fields = ('url', 'name') 

class OrderSerializer(serializers.ModelSerializer):
    class Meta: 
        model = Order 
        fields = '__all__' 

型号.py：

from __future__ import unicode_literals

from django.db import models 
from storage.choices import *

# Create your models here.
class Equipment(models.Model): 
    name = models.CharField(max_length=30) 
    fabricator = models.CharField(max_length=30, default='-') 
    storeplace = models.IntegerField() 
    labor = models.CharField(max_length=1, choices=labor_choices) 
    event = models.ManyToManyField('Order', blank = True, through= 'Assignment', through_fields=('Equipment', 'Order')) 
    max_quantity = models.IntegerField(default=1, null = True) 
    status = models.CharField(max_length=8, choices = STATUS_CHOICES, default = 'im Lager') 





    def __str__(self): 
        return self.name 




class client(models.Model): 
    firstname = models.CharField(max_length=30) 
    secondname = models.CharField(max_length=30) 
    email = models.EmailField() 
    post_code = models.IntegerField()
    city = models.CharField(max_length=30) 
    street= models.CharField(max_length=30) 



    def __str__(self):              
        return "%s %s" % (self.firstname, self.secondname)



class Order(models.Model): 
    name = models.CharField(max_length=30) 
    Type = models.CharField(
        max_length=2,
        choices=TYPE_CHOICES,
        default='Rental', 
        )
    city = models.CharField(max_length=30) 
    street= models.CharField(max_length=30)
    date = models.DateField() 
    GuestNumber = models.IntegerField() 
    description = models.TextField() 
    client = models.ForeignKey("client", on_delete=models.CASCADE, blank = True, null = True) 
    status = models.CharField(max_length=30, choices=order_choices, default='glyphicon glyphicon-remove') 

    def __str__(self): 
        return self.name

class Assignment(models.Model): 
    Equipment = models.ForeignKey('Equipment',  on_delete=models.CASCADE) 
    Order = models.ForeignKey('Order',  on_delete=models.CASCADE) 
    quantity = models.PositiveIntegerField(default=1)

提前谢谢。

Answer 1

我认为错误的原因：JSON parse error - No JSON object could be decoded是因为您忘记将.data放在@Ivan Semochkin solution:representation['assigment'] = AssignmentSerializer(instance.assigment_set.all(), many=True).data的第二行。

因此，我发现我将偶然发现Keyword Error: 'event'from line:representation = super(EquipmentSerializer, self).to_representation(instance)，因为equipmentserializer对象包含中介assignment_set，而不是event。

以下是我所做的@Ivan Semochkin解决方案的最终结果。如果我在实践中错了/不合适，请纠正我。

class EquipmentSerializer(serializers.ModelSerializer): 

    class Meta: 
        model = Equipment 
        fields = '__all__'

    def create(self, validated_data):
        order = Order.objects.get(pk=validated_data.pop('assignment_set').get('id'))
        instance = Equipment.objects.create(**validated_data)
        Assignment.objects.create(Order=order, Equipment=Equipment)
        return instance

    def to_representation(self, instance):
        representation = super(EquipmentSerializer, self).to_representation(instance)
        representation['assigment'] = AssignmentSerializer(instance.assigment_set.all(), many=True).data
        return representation 

如果我错了，请纠正我。我是新来的。

Answer 2

DRF不支持嵌套序列化程序的create方法。如果要在扩展布局中显示相关字段，而不仅仅是pks，则可以重写to_representation方法，而不是重写默认的mtm字段。您还应该重写create方法，因为mtm链接中有第三个模型：

class EquipmentSerializer(serializers.ModelSerializer): 

    class Meta: 
        model = Equipment 
        fields = '__all__'

    def create(self, validated_data):
        order = Order.objects.get(pk=validated_data.pop('event'))
        instance = Equipment.objects.create(**validated_data)
        Assignment.objects.create(Order=order, Equipment=instance)
        return instance

    def to_representation(self, instance):
        representation = super(EquipmentSerializer, self).to_representation(instance)
        representation['assigment'] = AssignmentSerializer(instance.assigment_set.all(), many=True).data
        return representation 

现在它将正确地保存传递pks列表的mtm字段，比如[1, 2, 3]，并且为了表示与mtm相关的模型，EquipmentSerializer将使用AssignmentSerializer。