Default value missing from nested serializer's validated data during update

Question

I'm trying to implement the update-method for nested representations in Django Rest Framework.

from django.db import models
from django.utils import timezone

from rest_framework import serializers


class Foo(models.Model):
    pass


class Bar(models.Model):
    foo = models.ForeignKey(Foo, on_delete=models.CASCADE, null=False, related_name='bars')
    name = models.CharField(max_length=255)
    create_date = models.DateField(null=False)


class BarSerializer(serializers.ModelSerializer):
    create_date = serializers.DateField(default=timezone.now)

    class Meta:
        model = Bar
        fields = ('name', 'create_date',)


class FooSerializer(serializers.ModelSerializer):
    bars = BarSerializer(many=True, required=False)

    def create(self, validate_data):
        bars_data = validated_data.pop('bars', [])
        foo = Foo.objects.create(**validated_data)

        for bar_data in bars_data:
            Bar.objects.create(foo=foo, **bar_data)

        return foo

    def update(self, instance, validated_data):
        bars_data = validated_data.pop('bars', None)
        if bars_data is not None:
            Bar.objects.filter(foo=instance).delete()
            for bar_data in bars_data:
                Bar.objects.create(foo=instance, **bar_data)

        return super().update(instance, validated_data)

    class Meta:
        model = Foo
        fields = ('bars',)

When creating an object I get the expected results, but whenever I want to update the bars list, without providing a create_date, I get an error saying that create_date cannot be None (which is correct):

django.db.utils.IntegrityError: NOT NULL constraint failed: myapp_bar.create_date

If I print bars_data inside update I can see that it doesn't get its default value from timezone.now. It isn't in the dictionary at all.

How do I access the default value here? It should have gone through the validation of the serializer, why isn't the default value added?

mehamasum · Accepted Answer · 2019-03-05 19:06:19Z

2

From docs:

The default is not applied during partial update operations. In the partial update case only fields that are provided in the incoming data will have a validated value returned.

So, you are going to have to add the value manually:

Bar.objects.create(foo=instance, **bar_data, create_date=timezone.now)

answered Mar 5, 2019 at 19:06

mehamasum

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Christian Buendia · Accepted Answer · 2019-03-05 20:09:33Z

0

You receive that error because in your Bar model the create_date attribute has null=False, if you want a default value prefer use auto_now or auto_now_add arguments in the created_date attr on your Bar model like this:

class Bar(models.Model):
    ...
    created_date = models.DateField(auto_now_add=True)

With this every time when you create a new instance of Bar the create_date will be added automatically.

answered Mar 5, 2019 at 20:09

Christian Buendia

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