Django Rest Framework: Disable save in update - django

Question

It is my first question here, after reading the similar questions I did not find what I need, thanks for your help.
I am creating a fairly simple API but I want to use best practices at the security level.
Requirement: There is a table in SQL Server with +5 million records that I should ONLY allow READ (all fields) and UPDATE (one field). This is so that a data scientist consumes data from this table and through a predictive model (I think) can assign a value to each record.
For this I mainly need 2 things:
That only one field is updated despite sending all the fields of the table in the Json (I think I have achieved it with my serializer).
And, where I have problems, is in disabling the creation of new records when updating one that does not exist.
I am using an UpdateAPIView to allow trying to allow a bulk update using a json like this (subrrogate_key is in my table and I use lookup_field to:
[
{
"subrrogate_key": "A1",
"class": "A"
},
{
"subrrogate_key": "A2",
"class": "B"
},
{
"subrrogate_key": "A3",
"class": "C"
},
]
When using the partial_update methods use update and this perform_update and this finally calls save and the default operation is to insert a new record if the primary key (or the one specified in lookup_field) is not found.
If I overwrite them, how can I make a new record not be inserted, and only update the field if it exists?
I tried:
Model.objects.filter (subrrogate_key = ['subrrogate_key']). Update (class = ['class])
Model.objects.update_or_create (...)
They work fine if all the keys in the Json exist, because if a new one comes they will insert (I don't want this).
P.S. I use a translator, sorry.

Answer 1

perform_update will create a new record if you passed a serializer that doesn't have an instance. Depending on how you wrote your view, you can simply check if there is an instance in the serializer before calling save in perform_update to prevent creating a new record:
def perform_update(self, serializer):
if not serializer.instance:
return
serializer.save()

Answer 2

Django implements that feature through the use of either force_update or update_fields during save().
https://docs.djangoproject.com/en/3.2/ref/models/instances/#forcing-an-insert-or-update
https://docs.djangoproject.com/en/3.2/ref/models/instances/#specifying-which-fields-to-save
https://docs.djangoproject.com/en/3.2/ref/models/instances/#saving-objects
In some rare circumstances, it’s necessary to be able to force the
save() method to perform an SQL INSERT and not fall back to doing an
UPDATE. Or vice-versa: update, if possible, but not insert a new row.
In these cases you can pass the force_insert=True or force_update=True
parameters to the save() method.
model_obj.save(force_update=True)
or
model_obj.save(update_fields=['field1', 'field2'])