Serialize Generic relationships with Django Rest Framework, with write support - django

Question

I have the following model:
class TaggedItem(models.Model):
tag = models.SlugField()
content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey('content_type', 'object_id')
I am trying to serialize this model in a way that I can assign the content object via an API endpoint
So far I have done this:
class TaggedItemSerializer(serializers.ModelSerializer):
content_object = serializers.RelatedField(read_only=True)
class Meta:
model = TaggedItem
However this is readonly. If I remove the read_only parameter, I must specify the queryset for the field. However, I have many different model types for this generic relationship. It seems like I am duplicating code if I specify all the possible model types both within the serializer and elsewhere in the model.
I could also set the content object through the object_id and content_type fields, but when I do this I get an error.
For example:
{
...
object_id: 1,
content_type: 'auth.User'
}
returns a 400 response with "detail": "JSON parse error - Expected object or value"
How can I make this content_object writable via the DRF api?

Answer 1

Override the .to_internal_value , .validate and .create methods like this:
from django.apps import apps
class TaggedItemSerializer(serializers.ModelSerializer):
class Meta:
model = TaggedItem
read_only_fields = ('content_type', 'object_id', 'content_object')
def to_internal_value(self, data):
object_id = data.pop('object_id')
content_type = data.pop('content_type')
ret = super(ConfigCalcSerializer, self).to_internal_value(data)
ret['object_id'] = object_id
ret['content_type'] = content_type
return ret
def validate(self, data):
object_id = data.pop('object_id')
content_type = data.pop('content_type')
Model = apps.get_model(content_type)
try:
content_object = Model.objects.get(id=object_id)
except Model.DoesNotExist:
raise serializers.ValidationError('Not found')
else:
data['content_object'] = content_object
return data
def create(self, validate_data):
return TaggedItem.objects.create(**validate_data)