Django Rest : Model field default value only while creating - django

Question

I have a model with 2 char-fields. I want to give them default values when a row is getting created. This default values are sort of IDs which depends on time. This ID should never change. i.e. the default value should not be applied when subsequent updates happen on the row.
I'm trying to use update_or_create but, the defaults while creating & updating are not same. How can I put these Ids only while creating and ignore while updating?
I'm referring to this answer but no luck.
EDIT:
Following is the code for reference:
Model:
class UsersModel(models.Model):
id = models.CharField(db_column="id", max_length=25, primary_key=True)
key = models.CharField(db_column="key", max_length=100)
a = models.CharField(db_column="a",max_length=25, null=True, blank=True)
b = models.BigIntegerField(db_column="b", null=True, blank=True)
Views:
def post(self, request, format=None):
UsersModel.objects.update_or_create(a="a_val",defaults={"b":"b_val"})
Here, I want the id & key to take default values when the row is being created. When it is being updated, only b should get updated as shown in above code.

Answer 1

The same can be achieved by overiding the save function of models.Model to ensure that an id and key is given a value(default) in your case if the model is being created.
class UsersModel(models.Model):
id = models.CharField(db_column="id", max_length=25, primary_key=True)
key = models.CharField(db_column="key", max_length=100)
a = models.CharField(db_column="a",max_length=25, null=True, blank=True)
b = models.BigIntegerField(db_column="b", null=True, blank=True)
def save():
if not self.id:
self.id=get_default('id')
if not self.key:
self.key=get_default('key')
super(UsersModel, self).save(*args, **kwargs)
This will ensure when you create and update, the first time save is called, these variables are assigned a default value before getting saved in the db.