Serializer field for side effect model django rest framework - django-models

Question

I have a django.db.models.Model A whose instances are created in a rest_framework.serializers.ModelSerializer from POST requests.
Depending on the data being sent in the POST, I would like to create one of several other "addon" models, let's say B or C, which I link to the original through a django.db.models.OneToOneField:
from django.db import models
class A(models.Model):
some_field = models.CharField()
class B(models.Model):
a = models.OneToOneField(A, related_name='addon', on_delete=models.CASCADE)
class C(models.Model):
a = models.OneToOneField(A, related_name='addon', on_delete=models.CASCADE)
What I would like to is to have a serializer which validates the incoming data, including some string indicating which addon to use. The serializer then creates the model instance of A and based on this creates the addon model.
I do not want to create a utility field in model A used to determine which addon to use, I would like to create the model directly using the instance of model A and information from the POST itself.
At the same time when accessing the data through a get, I would like to return the original string used to determine which addon to use.
What I have come up with so far:
from rest_framework import serializers
str2model = {'b': B, 'c': C}
class AddonField(serializers.Field):
def to_representation(self, value):
# I completely ignore "value" as no "internal value" is set in "to_internal_value"
myvalue = self.parent.instance.addon
for addon_name, addon_class in str2model.items():
if isinstance(myvalue, addon_class):
return addon_name
def to_internal_value(self, data):
# I create the "internal value" after "A" instance is created, thus here I do nothing?
return data
class ASerializer(serializers.ModelSerializer):
some_field = serializers.CharField()
the_addon = AddonField()
def validate_the_addon(self, value): # here addon is a string
if value in str2model.keys():
return value
def create(self, validated_data):
addon_name = validated_data.pop('the_addon')
addon_class = str2model[addon]
a = super(ASerializer, self).create(validated_data)
addon_class.objects.create(a=a)
return a
class Meta:
model = A
fields = ["some_field", "the_addon"]
When testing this I get:
AttributeError: Got AttributeError when attempting to get a value for field `the_addon` on serializer `ASerializer`.
The serializer field might be named incorrectly and not match any attribute or key on the `A` instance.
Original exception text was: 'A' object has no attribute 'the_addon'.
How can I temporarily store the_addon in the serializer until the A instance has been created?

Answer 1

This is how I would typically approach it
# Serializer
class ASerializer(serializers.Serializer):
some_field = serializers.CharField()
addon_b = serializers.CharField(required=False, allow_null=True)
addon_c = serializers.CharField(required=False, allow_null=True)
def create(self, validated_data):
addon_b = validated_data.pop('addon_b')
addon_c = validated_data.pop('addon_c')
a = A.objects.create(some_field=validated_data['some_field'])
if addon_b:
B.objects.create(a=a)
if addon_c:
C.objects.create(a=a)
return a
You can do other validations if necessary.
class TestAPIView01(generics.CreateAPIView):
permission_classes = {}
serializer_class = serializers.ASerializer
queryset = A.objects.all()
Also, look at the related_name on B and C you may want to consider making them different, as that might throw an error in the future. Cheers