[Django]-The `.create()` method does not support writable nested fields by default.

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

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

I think that the cause for error: JSON parse error - No JSON object could be decoded is because you forgot to put .data at the 2nd line from @Ivan Semochkin solution: representation['assigment'] = AssignmentSerializer(instance.assigment_set.all(), many=True).data.

Thus I find out that I will stumble upon Keyword Error: 'event' from line: representation = super(EquipmentSerializer, self).to_representation(instance) since the EquipmentSeralizer object contain the intermediary assignment_set instead of event.

Here are the end result adapting from @Ivan Semochkin solution I do. Correct me if I’m wrong/inappropriate in practice.

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 

Please correct me if I’m wrong. I’m new to Django.