Python Django Rest Framework: The `.create()` method does not support writable nested fields by default

Question

I am using django rest framework to create an api endpoint. I am using the default user model django offers. I need to create a post which uses the user as a foreign key. A user called "author" in the post can have multiple posts.

This is an example of a post json.

[
    {
        "author": {
            "id": 1,
            "username": "sorin"
        },
        "title": "First Post",
        "description": "Hello World!",
        "created_at": "2020-08-05T14:20:51.981163Z",
        "updated_at": "2020-08-05T14:20:51.981163Z"
    }
]

This is the model.

class Post(models.Model):
    author = models.ForeignKey(User, on_delete=models.CASCADE)
    title = models.CharField(max_length=255)
    description = models.TextField()
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    def __str__(self):
        return self.title

This is the serializer.

class UserSerializer(serializers.ModelSerializer):

    class Meta:
        model = User
        fields = ('id', 'username')


class PostSerializer(serializers.HyperlinkedModelSerializer):

    author = UserSerializer()

    class Meta:
        model = Post
        fields = ('author', 'title', 'description', 'created_at', 'updated_at')

I am getting the error "The .create() method does not support writable nested fields by default." when trying to make a post request using a "username", "title" and "description".

Any help to how to solve this?

You can't directly save nested data. This may help.stackoverflow.com/questions/41394761/… — Sagar Adhikari
– Sagar Adhikari, Commented Aug 5, 2020 at 16:07
You can use a 3rd party library to get nested, writable serializers github.com/beda-software/drf-writable-nested — Ross Rogers
– Ross Rogers, Commented Aug 5, 2020 at 16:28

loicgasser · Accepted Answer · 2020-08-05 21:06:03Z

1

I like hooking in the create function of the serializer for these kind of use cases. Make sure your UserSerializer is set to read_only=True.

class PostSerializer(serializers.HyperlinkedModelSerializer):

    author = UserSerializer(read_only=True)

    class Meta:
        model = Post
        fields = ('author', 'title', 'description', 'created_at', 'updated_at')

    def create(self, validated_data):
       request = self.context['request']
       author_data = request.data.get('author')
       if author is None or not isinstance(author.get('id'), int):
           raise ValidationError({'author': ['This field is invalid.']})
       author_instance = get_object_or_404(User, id=author.get('id'))
       return Post.objects.create(author=author_instance, **validated_data)

answered Aug 5, 2020 at 21:06

loicgasser

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