Should literal db_default values skip model validation?

Django Internals ORM
1

We skipped model validation of db_default values to fix a crash in 5.0.4 (PR).

For generated values, that makes sense, and aligns with GeneratedField.

But for literal values, I find that surprising. Can’t we resolve the db_default expression to find out if it’s a literal value, and validate that?

Skipping model validation like this is, I think, unexpected – it’s not difficult for me to imagine db_defaults and validation requirements either getting out of sync or being intentionally different (Django-writes being subject to stricter requirements than direct-writes):

from django.core import validators
from django.db import models


class Article(models.Model):
    slug = models.SlugField(db_default="not*sluggable")

    def __str__(self):
        return self.slug

def run():
    instance = Article()
    instance.full_clean()  # succeeds
    instance.save()
    print(f'Created: {instance}')
    # instance.full_clean()  # fails

Direct validator usage also doesn’t work:

def run():
    instance = Article()
    instance._meta.get_field("slug").run_validators(instance.slug)

  File "/usr/local/lib/python3.12/site-packages/django/core/validators.py", line 503, in clean
    return len(x)
           ^^^^^^
TypeError: object of type 'DatabaseDefault' has no len()
2

Just noticed that @shangxiao came to a similar conclusion here that we should unwrap literal values and validate them for form fields, which I acknowledge is slightly different.