Override Django (DRF) Serializer object GET - django

Question

I'm trying to "inject" some raw sql into my DRF nested Serializer:
# SERIALIZERS
class CarSerializer(serializers.ModelSerializer):
class Meta:
model = Car
fields = '__all__'
class DriverSerializer(serializers.ModelSerializer):
car = CarSerializer() # <--- here I don't want to get the Car object but rather inject a raw sql.
class Meta:
model = Driver
fields = '__all__'
The SQL injection is needed to request for a specific version of the data since I'm using MariaDB versioning tables but this is not relevant. How do I override the method that gets the object from CarSerializer? Thank you.

Answer 1

This is untested but I think you want to override the __init__ in DriverSerializer and then load the result of your raw SQL via data, something like this:
class DriverSerializer(serializers.ModelSerializer):
[...]
def __init__(self, *args, **kwargs):
super(DriverSerializer, self).__init__(*args, **kwargs)
name_map = {'column_1': 'obj_attr_1', 'column_2': 'obj_attr_1', 'pk': 'id'}
raw = Car.objects.raw('SELECT ... FROM ...', translations=name_map)
data = {k: getattr(raw[0], k) for k in name_map.keys()}
self.car = CarSerializer(data=data)

Answer 2

You could define method under your model to get related Car
class Car(models.Model):
def current_car(self):
return Car.objects.raw('SELECT ... FROM ...')[0]
Then in serializer you could reuse following method
class DriverSerializer(serializers.ModelSerializer):
car = CarSerializer(source="current_car")
class Meta:
model = Driver
fields = (...)

Answer 3

Thank you everyone for your answers, I managed to make it work although my solution is not as clean as the one suggested from #yvesonline and #iklinak:
I first checked the official DRF documentation on overriding serializers: https://www.django-rest-framework.org/api-guide/serializers/#overriding-serialization-and-deserialization-behavior
In particular I was interested in the overriding of the method: .to_representation(self, instance) that controls the fetching of the object from the database:
from datetime import datetime as dt
from collections import OrderedDict
from rest_framework.relations import PKOnlyObject
from rest_framework.fields import SkipField, empty
def __init__(
self, instance=None, data=empty, asTime=str(dt.now()), **kwargs):
self.asTime = asTime
self.instance = instance
if data is not empty:
self.initial_data = data
self.partial = kwargs.pop('partial', False)
self._context = kwargs.pop('context', {})
kwargs.pop('many', None)
super().__init__(**kwargs)
def to_representation(self, instance):
# substitute instance with my raw query
# WARNING: self.asTime is a custom variable, check the
# __init__ method above!
instance = Car.objects.raw(
'''
select * from db_car
for system_time as of timestamp %s
where id=%s;
''', [self.asTime, instance.id])[0]
ret = OrderedDict()
fields = self._readable_fields
for field in fields:
try:
attribute = field.get_attribute(instance)
except SkipField:
continue
check_for_none = attribute.pk if isinstance(
attribute, PKOnlyObject) else attribute
if check_for_none is None:
ret[field.field_name] = None
else:
ret[field.field_name] = field.to_representation(attribute)
return ret
You can find the original code here: https://github.com/encode/django-rest-framework/blob/19655edbf782aa1fbdd7f8cd56ff9e0b7786ad3c/rest_framework/serializers.py#L335
Then finally in the DriverSerializer class:
class DriverSerializer(serializers.ModelSerializer):
car = CarSerializer(asTime='2021-02-05 14:34:00')
class Meta:
model = Driver
fields = '__all__'