DRF Writable Nested

This is a writable nested model serializer for Django REST Framework which allows you to create/update your models with related nested data.

The following relations are supported:

• OneToOne (direct/reverse)
• ForeignKey (direct/reverse)
• ManyToMany (direct/reverse excluding m2m relations with through model)
• GenericRelation (this is always only reverse)

Requirements

• Python (3.8, 3.9, 3.10, 3.11, 3.12)
• Django (4.2, 5.0)
• djangorestframework (3.14+)

Installation

pip install drf-writable-nested

Usage

For example, for the following model structure:

from django.db import models


class Site(models.Model):
    url = models.CharField(max_length=100)


class User(models.Model):
    username = models.CharField(max_length=100)


class AccessKey(models.Model):
    key = models.CharField(max_length=100)


class Profile(models.Model):
    sites = models.ManyToManyField(Site)
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    access_key = models.ForeignKey(AccessKey, null=True, on_delete=models.CASCADE)


class Avatar(models.Model):
    image = models.CharField(max_length=100)
    profile = models.ForeignKey(Profile, related_name='avatars', on_delete=models.CASCADE)

We should create the following list of serializers:

from rest_framework import serializers
from drf_writable_nested.serializers import WritableNestedModelSerializer


class AvatarSerializer(serializers.ModelSerializer):
    image = serializers.CharField()

    class Meta:
        model = Avatar
        fields = ('pk', 'image',)


class SiteSerializer(serializers.ModelSerializer):
    url = serializers.CharField()

    class Meta:
        model = Site
        fields = ('pk', 'url',)


class AccessKeySerializer(serializers.ModelSerializer):

    class Meta:
        model = AccessKey
        fields = ('pk', 'key',)


class ProfileSerializer(WritableNestedModelSerializer):
    # Direct ManyToMany relation
    sites = SiteSerializer(many=True)

    # Reverse FK relation
    avatars = AvatarSerializer(many=True)

    # Direct FK relation
    access_key = AccessKeySerializer(allow_null=True)

    class Meta:
        model = Profile
        fields = ('pk', 'sites', 'avatars', 'access_key',)


class UserSerializer(WritableNestedModelSerializer):
    # Reverse OneToOne relation
    profile = ProfileSerializer()

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

Also, you can use NestedCreateMixin or NestedUpdateMixin from this package if you want to support only create or update logic.

For example, we can pass the following data with related nested fields to our main serializer:

data = {
    'username': 'test',
    'profile': {
        'access_key': {
            'key': 'key',
        },
        'sites': [
            {
                'url': 'http://google.com',
            },
            {
                'url': 'http://yahoo.com',
            },
        ],
        'avatars': [
            {
                'image': 'image-1.png',
            },
            {
                'image': 'image-2.png',
            },
        ],
    },
}

user_serializer = UserSerializer(data=data)
user_serializer.is_valid(raise_exception=True)
user = user_serializer.save()

This serializer will automatically create all nested relations and we receive a complete instance with filled data.

user_serializer = UserSerializer(instance=user)
print(user_serializer.data)

{
    'pk': 1,
    'username': 'test',
    'profile': {
        'pk': 1,
        'access_key': {
            'pk': 1,
            'key': 'key'
        },
        'sites': [
            {
                'pk': 1,
                'url': 'http://google.com',
            },
            {
                'pk': 2,
                'url': 'http://yahoo.com',
            },
        ],
        'avatars': [
            {
                'pk': 1,
                'image': 'image-1.png',
            },
            {
                'pk': 2,
                'image': 'image-2.png',
            },
        ],
    },
}

It is also possible to pass through values to nested serializers from the call to the base serializer's save method. These kwargs must be of type dict. E g:

# user_serializer created with 'data' as above
user = user_serializer.save(
    profile={
        'access_key': {'key': 'key2'},
    },
)
print(user.profile.access_key.key)

'key2'

Note: The same value will be used for all nested instances like default value but with higher priority.

Testing

To run unit tests, run:

# Setup the virtual environment
python3 -m venv envname
source envname/bin/activate

pip install django
pip install django-rest-framework
pip install -r requirements.txt

# Run tests
py.test

Known problems with solutions

Validation problem for nested serializers with unique fields on update

We have a special mixin UniqueFieldsMixin which solves this problem. The mixin moves UniqueValidator's from the validation stage to the save stage.

If you want more details, you can read related issues and articles: #1 http://www.django-rest-framework.org/api-guide/validators/#updating-nested-serializers

Example of usage:

class Child(models.Model):
    field = models.CharField(unique=True)


class Parent(models.Model):
    child = models.ForeignKey('Child')


class ChildSerializer(UniqueFieldsMixin, serializers.ModelSerializer):
    class Meta:
        model = Child


class ParentSerializer(NestedUpdateMixin, serializers.ModelSerializer):
    child = ChildSerializer()

    class Meta:
        model = Parent

Note: UniqueFieldsMixin must be applied only on serializer which has unique fields.

Mixin ordering

When you are using both mixins (UniqueFieldsMixin and NestedCreateMixin or NestedUpdateMixin) you should put UniqueFieldsMixin ahead.

For example:

class ChildSerializer(UniqueFieldsMixin, NestedUpdateMixin,
        serializers.ModelSerializer):

Update problem for nested fields with form-data in PATCH and PUT methods

There is a special problem while we try to update any model object with nested fields within it via PUT or PATCH using form-data we can not update it. And it complains about fields not provided. So far, we came to know that this is also a problem in DRF. But we can follow a tricky way to solve it at least for now. See the below solution about the problem

If you want more details, you can read related issues and articles: #106 encode/django-rest-framework#7262 (comment)

Example:

# Models
class Voucher(models.Model):
    voucher_number = models.CharField(verbose_name="voucher number", max_length=10, default='')
    image = models.ImageField(upload_to="vouchers/images/", null=True, blank=True)

class VoucherRow(models.Model):
    voucher = models.ForeignKey(to='voucher.Voucher', on_delete=models.PROTECT, verbose_name='voucher',
                                related_name='voucherrows', null=True)
    account = models.CharField(verbose_name="fortnox account number", max_length=255)
    debit = models.DecimalField(verbose_name="amount", decimal_places=2, default=0.00, max_digits=12)
    credit = models.DecimalField(verbose_name="amount", decimal_places=2, default=0.00, max_digits=12)
    description = models.CharField(verbose_name="description", max_length=100, null=True, blank=True)

# Serializers for these models
class VoucherRowSerializer(WritableNestedModelSerializer):
    class Meta:
        model = VoucherRow
        fields = ('id', 'account', 'debit', 'credit', 'description',)


class VoucherSerializer(serializers.ModelSerializer):
    voucherrows = VoucherRowSerializer(many=True, required=False, read_only=True)
    class Meta:
        model = Voucher
        fields = ('id', 'participants', 'voucher_number', 'voucherrows', 'image')

Now if you want to update Voucher with VoucherRow and voucher image then you need to do it using form-data via PUT or PATCH request where your voucherrows fields are nested field. With the current implementation of the drf-writable-nested doesn't update it. Because it does not support something like-

voucherrows[1].account=1120
voucherrows[1].debit=1000.00
voucherrows[1].credit=0.00
voucherrows[1].description='Debited from Bank Account' 
voucherrows[2].account=1130
voucherrows[2].debit=0.00
voucherrows[2].credit=1000.00
voucherrows[2].description='Credited to Cash Account'

This is not supported at least for now. So, we can achieve the result in a different way. Instead of sending the array fields separately in this way we can convert the whole fields along with values in a json string like below and set it as value to the field voucherrows.

"[{\"account\": 1120, \"debit\": 1000.00, \"credit\": 0.00, \"description\": \"Debited from Bank Account\"}, {\"account\": 1130, \"debit\": 0.00, \"credit\": 1000.00, \"description\": \"Credited to Cash Account\"}]"

Now it'll be actually sent as a single field value to the application for the field voucherrows. From your views you need to parse it like below before sending it to the serializer-

class VoucherViewSet(viewsets.ModelViewSet):
    serializer_class = VoucherSerializer
    queryset = serializer_class.Meta.model.objects.all().order_by('-created_at')
    
    def update(self, request, *args, **kwargs):
        request.data.update({'voucherrows': json.loads(request.data.pop('voucherrows', None))})
        return super().update(request, *args, **kwargs)

Now, you'll get the voucherrows field with data in the right format in your serializers. Similar approach will be also applicable for generic views for django rest framework

Authors

2014-2022, beda.software