Unable to get a non-model field in the validated_d

I have an ItemCollection and Items in my Django models and I want to be able to remove Items from the collection through the UI. In a REST PUT request I add an extra boolean field deleted for each Item to signal that an Item should be deleted.

The correct way to handle this seems to be in the update method of the Serializer. My problem is that this non-model deleted field gets removed during validation, so it is not available anymore. Adding deleted as a SerializerMethodField did not help. For now I get my deleted information from the initial_data attribute of the Serializer, but that does not feel right.

My current example code is below. Does anybody know a better approach?

Models:

    class ItemCollection(models.Model):
        description = models.CharField(max_length=256)


    class Item(models.Model):
        collection = models.ForeignKey(ItemCollection, related_name="items")

Serializers:

    from django.shortcuts import get_object_or_404
    from rest_framework.views import APIView
    from rest_framework.response import Response
    from rest_framework import serializers
    from models import Item, ItemCollection


    class ItemSerializer(serializers.ModelSerializer):

        class Meta:
            model = Item


    class ItemCollectionSerializer(serializers.ModelSerializer):

        items = ItemSerializer(many=True, read_only=False)

        class Meta:
            model = ItemCollection

        def update(self, instance, validated_data):
            instance.description = validated_data['description']
            for item, item_obj in zip(
                   self.initial_data['items'], validated_data['items']):
                if item['delete']:
                    instance.items.filter(id=item['id']).delete()
            return instance


    class ItemCollectionView(APIView):

        def get(self, request, ic_id):
            item_collection = get_object_or_404(ItemCollection, pk=ic_id)
            serialized = ItemCollectionSerializer(item_collection).data
            return Response(serialized)

        def put(self, request, ic_id):
            item_collection = get_object_or_404(ItemCollection, pk=ic_id)
            serializer = ItemCollectionSerializer(
               item_collection, data=request.data)
            if serializer.is_valid(raise_exception=True):
                serializer.save()
            return Response(serializer.data)

And an example of the json in the PUT request:

    {
        "id": 2,
        "items": [
            {
                "id": 3,
                "collection": 2,
                "delete": true
            }
        ],
        "description": "mycoll"
    }

You can add non-model fields back by overwriting the to_internal_value fn:

def to_internal_value(self, data):
    internal_value = super(MySerializer, self).to_internal_value(data)
    my_non_model_field_raw_value = data.get("my_non_model_field")
    my_non_model_field_value = ConvertRawValueInSomeCleverWay(my_non_model_field_raw_value)
    internal_value.update({
        "my_non_model_field": my_non_model_field_value
    })
    return internal_value

Then you can process it however you want in create or update.