The `.create()` method does not support writable nested fields by default.

Question

I have a big problem regarding the serialization of a Many to Many relationship with intermediate model in DRF: If the request method is get everything works perfectly. But as soon as i try to POST or PUT Data to the API I get the following Error:

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.

I am not really sure how to write proper create and update functions and i don´t really understand it, how it is explained in the documentation.

Code:

views.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  

serializers.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__' 

models.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)

Thanks in Advance.

Answer 1

DRF does not support create method for nested serializers. If you want to show related fields in an extended layout and not only with pks then you can override the to_representation method instead of rewriting default mtm field. You should also override a create method, because of the third model in mtm link:

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 

Now it'll save mtm fields properly passing list of pks, like [1, 2, 3] and for representation of that mtm related model, EquipmentSerializer will use AssignmentSerializer.

Answer 2

Maybe for most people who is having the same issue, this question is a quite long.

The short answer is that DRF does not support natively create method for nested serializers. so what to do?

Simply overriding the default behaviour. Check out a full example in the Official DRF docs

Answer 3

I had a similar problem but with the update() method ...

The solution was simple thanks to this thread: https://github.com/beda-software/drf-writable-nested/issues/104...

All I had to do was installing the library pip install drf-writable-nested and import it:

from drf_writable_nested import WritableNestedModelSerializer

the code should look like this:

(credit to: https://github.com/Leonardoperrella)

--serializers.py--

from drf_writable_nested import WritableNestedModelSerializer

class ProductsSerializer(serializers.ModelSerializer):
    class Meta:
        model = Products
        fields = ('name', 'code', 'price')

class VendorsSerializer(WritableNestedModelSerializer,
                        serializers.ModelSerializer):
    products = ProductsSerializer(source='vendor', many=True)
    class Meta:
        model = Vendors
        fields = ('name', 'cnpj', 'city', 'products')

--models.py--

class Vendors(models.Model):
    name = models.CharField('Name', max_length=50)
    cnpj = models.CharField('CNPJ', max_length=14, unique=True, validators=[validate_cnpj])
    city = models.CharField('City', max_length=100, blank=True)

    class Meta:
        verbose_name = "Vendor"
        verbose_name_plural = "Vendors"

    def __str__(self):
        return self.name


class Products(models.Model):
    name = models.CharField('Name', max_length=60)
    code = models.CharField('Code', max_length=13)
    price = models.DecimalField('Price',
                                max_digits=15,
                                decimal_places=2,
                                default=0.00,
                                validators=[MinValueValidator(Decimal("0.01"))])
    vendor = models.ForeignKey('Vendors', on_delete=models.CASCADE, related_name='vendor')

    class Meta:
        verbose_name = "Product"
        verbose_name_plural = "Products"

    def __str__(self):
        return self.name