#35731: Extend documentation about db_default and DatabaseDefault
-------------------------------------+-------------------------------------
Reporter: Kyle Bebak | Owner:
Type: | YashRaj1506
Cleanup/optimization | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
> I think foo.val should be None, not an instance of DatabaseDefault
(which can be stored elsewhere on the model instance).
Without getting into all the details there are complications in doing that
as `None` is already used to denote `NULL` and a field with a `db_default`
could also be marked `null=True` and assigned `None` before creation to
take precedence over `db_default`
{{{#!python
class Author(models.Model):
favorite_color = models.CharField(db_default="green", null=True)
author = Author()
assert isinstance(author, DatabaseDefault)
author.favorite_color = None
author.save()
author.refresh_from_db()
assert author.favorite_color is None
}}}
In other words we can't use `None` as a sentinel denote a fallback to
`db_default` as it might be explicitly assigned as a desired value.
> If default is passed to the field, then val has a type of int. If it's
not, then its type is int | None. In my proposal above, passing db_default
wouldn't change the type of foo, whereas the current implementation means
its type is int | DatabaseDefault, int | None, or int, depending on what
combo of default and db_default is passed.
That's a bad assumption unfortunately, every property associated with a
field are `| Expression[F]` where `F` is bound to the field type and
represents its `output_field`. In other words, `IntegerField(null=True)`
results in a descriptor that is `int | None | Expression[IntegerField]`
and not simply `int | None`.
--
Ticket URL: <https://code.djangoproject.com/ticket/35731#comment:5>
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