DRF: serializer for an order with nested items (creation and validation)

# ======================================================================
# Q: Write a serializer for an order with nested items (creation and
# validation).
#
# Nested writable serializers need your own create()/update(). The logic (stock,
# prices, transaction) goes in a service, not in the serializer. The price does
# not come from the client, the server calculates it.
# ======================================================================

class OrderItemSerializer(serializers.ModelSerializer):
    class Meta:
        model = OrderItem
        fields = ['product', 'quantity']

class OrderCreateSerializer(serializers.ModelSerializer):
    items = OrderItemSerializer(many=True)

    class Meta:
        model = Order
        fields = ['id', 'items']

    def validate_items(self, items):
        if not items:
            raise serializers.ValidationError('Order must have at least one item.')
        skus = [i['product'].pk for i in items]
        if len(skus) != len(set(skus)):
            raise serializers.ValidationError('Duplicate products.')
        return items

    def create(self, validated_data):
        customer = self.context['request'].user.customer
        return services.place_order(customer, validated_data['items'])  # atomic + stock