python - Django-Rest-Framework. Updating nested object

Question

I'm having a problem of updating a nested object.

So I have a model which structure is similar to this one:

class Invoice(models.Model):
    nr = models.CharField(max_length=100)
    title = models.CharField(max_length=100)

class InvoiceItem(models.Model):
    name = models.CharField(max_length=100)
    price = models.FloatField()
    invoice = models.ForeignKey(Invoice, related_name='items')

I need to create child objects from parent, and what I mean by that, is to create InvoiceItems directly when creating an Invoice object. For this purpose, I've wrote the following serializers:

class InvoiceItemSerializer(serializers.ModelSerializer):
    invoice = serializers.PrimaryKeyRelatedField(queryset=Invoice.objects.all(), required=False)
    class Meta:
        model = InvoiceItem


class InvoiceSerializer(serializers.ModelSerializer):
    items = InvoiceItemSerializer(many=True)

    class Meta:
        model = Invoice

    def create(self, validated_data):
        items = validated_data.pop('items', None)
        invoice = Invoice(**validated_data)
        invoice.save()
        for item in items:
            InvoiceItem.objects.create(invoice=invoice, **item)
        return invoice

Up till now, the create/read/delete methods work perfectly, except the update. I think the below logic should be correct, but it misses something.

def update(self, instance, validated_data):
    instance.nr = validated_data.get('nr', instance.nr)
    instance.title = validated_data.get('title', instance.title)
    instance.save()

    # up till here everything is updating, however the problem appears here.
    # I don't know how to get the right InvoiceItem object, because in the validated
    # data I get the items queryset, but without an id.

    items = validated_data.get('items')
    for item in items:
        inv_item = InvoiceItem.objects.get(id=?????, invoice=instance)
        inv_item.name = item.get('name', inv_item.name)
        inv_item.price = item.get('price', inv_item.price)
        inv_item.save()

    return instance

Any help would be really appreciated.

question from:https://stackoverflow.com/questions/37240621/django-rest-framework-updating-nested-object

Answer 1

This is the way I've accomplished the task:

I've added an id field to the InvoiceItemSerializer

class InvoiceItemSerializer(serializers.ModelSerializer):
    ...
    id = serializers.IntegerField(required=False)
    ...

And the update method for the InvoiceSerializer

def update(self, instance, validated_data):
    instance.nr = validated_data.get('nr', instance.nr)
    instance.title = validated_data.get('title', instance.title)
    instance.save()

    items = validated_data.get('items')

    for item in items:
        item_id = item.get('id', None)
        if item_id:
            inv_item = InvoiceItem.objects.get(id=item_id, invoice=instance)
            inv_item.name = item.get('name', inv_item.name)
            inv_item.price = item.get('price', inv_item.price)
            inv_item.save()
        else:
            InvoiceItem.objects.create(account=instance, **item)

    return instance

Also in the create method I'm popping the id if it is passed.