How do I write to nested parent in Django Serializer - django

Question

I'm using Django serializers to create an API which I both read from and write to.
Models:
class Biomarker(models.Model):
low_value_description = models.TextField(blank=True, null=True)
high_value_description = models.TextField(blank=True, null=True)
class BiomarkerReading(models.Model):
biomarker = models.ForeignKey(Biomarker, on_delete=models.CASCADE)
test = models.ForeignKey(Test, on_delete=models.CASCADE)
value = models.DecimalField(max_digits=30, decimal_places=8, default=0)
Serializer:
class BiomarkerReadingSerializer(serializers.ModelSerializer):
class Meta:
model = BiomarkerReading
fields = (
'id', 'test', 'biomarker', 'value'
)
JSON format:
{
"id": 617188,
"test" 71829,
"biomarker": 32,
"value": 0.001
}
The above all works, and I can read and write to it with that JSON format. However I now need to add some fields from the parent model so the response looks like this:
{
"id": 617188,
"test" 71829,
"biomarker": {
"id": 32,
"low_value_description": "All good",
"high_value_description": "You will die",
},
"value": 0.001
}
I have got the read part working using these Serializers:
class BiomarkerDescriptionSerializer(serializers.ModelSerializer):
class Meta:
model = Biomarker
fields = ('id', 'low_value_description', 'high_value_description')
class BiomarkerReadingSerializer(serializers.ModelSerializer):
biomarker = BiomarkerDescriptionSerializer()
class Meta:
model = BiomarkerReading
fields = (
'id', 'test', 'biomarker', 'value'
)
However I can't find a way to write to it using the old json format (with "biomarker": 32 in the JSON).
I thought I would need to do something in validate or create, but I get a 400 response before it even hits those methods:
class BiomarkerReadingSerializer(serializers.ModelSerializer):
... # as above
def validate(self, data):
print('validate') # Doesn't print
data = super().validate(data)
return data
def create(self, validated_data):
print('create') # Doesn't print
return super().create(validated_data)
The example in the docs for writable-nested-serializers and the other examples I've found on SO only discuss the case of creating child records while writing to the parent record serializer, not the other way around.
I do not want to create a parent Biomarker via the API, I just need to be able to reference it by pk/id in the incoming JSON like before.
I don't mind if I have to change the names of keys to something like this for incoming:
{
"id": 617188,
"test" 71829,
"biomarker_id": 32,
"value": 0.001
}
Or something like this for the response:
{
"id": 617188,
"test" 71829,
"biomarker": 32,
"descriptions": {
"low_value_description": "All good",
"high_value_description": "You will die",
},
"value": 0.001
}
If that makes it easier.

Answer 1

I hate answering my own question, but the solution was to subclass serializers.RelatedField, which is explained in advanced-serializer-usage).
This lets you separately control how the value(instance) is represented as serialised, and how the serialised data is used to retrieve or create a value (instance).
class BiomarkerDescriptionSerializer(serializers.RelatedField):
def to_representation(self, value):
data = {}
for field in ('id', 'low_value_description', 'high_value_description'):
data[field] = getattr(value, field)
return data
def to_internal_value(self, data):
return Biomarker.objects.get(id=data)
def get_queryset(self, *args):
pass
class BiomarkerReadingSerializer(serializers.ModelSerializer):
biomarker = BiomarkerDescriptionSerializer()
...
The overridden get_queryset is required, even though it does nothing.
This means data going in looks like this:
{
"id": 617188,
"test" 71829,
"biomarker": 32,
"value": 0.001
}
Yet the data going out looks like this:
{
"id": 617188,
"test" 71829,
"biomarker": {
"id": 32,
"low_value_description": "All good",
"high_value_description": "You will die",
},
"value": 0.001
}
Thank you to those who offered answers, much appreciated

Answer 2

Using depth = 1 option will solve your problem.
class BiomarkerReadingSerializer(serializers.ModelSerializer):
class Meta:
model = BiomarkerReading
fields = (
'id', 'test', 'biomarker', 'value'
)
depth = 1
Take a look: https://www.django-rest-framework.org/api-guide/serializers/#specifying-nested-serialization