Python – Access primary key/id values during Django record creation

For my model, I use the default auto-incrementing PrimaryKey/id field. I also have a field called serialNumber which, by design, should reference id and generate its own value based on the id value.
For example, if the id is 1234, then the serialNumber field should be a string ‘00001234'.

class MyModel(models. Model):
    ...
    serialNumber = models. TextField(max_length=8)
    ...

I

have no problem updating serialNumber after creating a record in the database, but my specific situation requires serialNumber to be set along with the initial creation of the object, because after it I return the record via the REST API.

I tried using signals, but pre_save returned None

@receiver(pre_save, sender=MyModel)
def my_pre_save(sender, instance, **kwargs):
    print(instance.id)

On the other hand, if I do:

@receiver(post_save, sender=MyModel)
def my_post_save(sender, instance, **kwargs):
    print(instance.id)

I

received the value for id, but it was too late because after .save() my REST API returned a response with the created object, and I didn’t want to add code to manage serialNumber to the serializer.

Overriding the .save() method for MyModel also doesn’t work:

    def save(self, *args, **kwargs):
        print(self.id)
        super(MyModel, self).save(*args, **kwargs)

The Id field still returns None.

Does anyone know how to access the id value during initial record creation?

Well, you can’t access the ID before saving it.
The ID will be generated by the database, so you have to call the save method so that django sends the data to the database, and then you will have your ID

This is also explained in the Django documentation:

Auto-incrementing primary keys

If you do need access to a uniquely identified ID, you can create one manually.

You can add a field named my_id and then give it a unique value before saving the data (you just need to check if the value already exists, and if it does, just create another one). In this way, you will be able to get a unique ID with which you can find the row in the future.