Support create defaults for update_or_create

[Timothy Schilling]

I proposed the idea of extending update_or_create to support specifying a different set of defaults for the create operation on the [forum](​https://forum.djangoproject.com/t/feature-idea-update-or-create-to-allow-different-defaults-for-create-and-update-operations/18300/15). There seems to be consensus it's a positive add to Django.

Adam raised concerns with my proposed approach of adding a create_defaults parameter to the function since this would conflict with any fields on a model named, create_defaults. Jeff did a code search on github for that term and didn't find any matches. I suspect if someone where using a field named create_defaults, it would be a JSON or object type field. Those don't seem like reasonable candidates to be part of a UniqueConstraint, which should be underlying the look-up arguments to update_or_create.

I do like the idea of having a separate parameter for create_defaults, but if we must preserve 100% backwards compatibility, Adam's suggestion of having defaults be set to another object makes the most sense.

My blocking question is, which approach should I take?

From the forum post:

I’ve run into a use-case in which it’d be helpful to have the ability to specify a different set of defaults for the update operation compared to the create operation. While I don’t expect my particular use case to translate, here’s a more generic one.
Given the following Record model:

class Record(models.Model):
    some_id = models.CharField(unique=True)
    created_by = models.ForeignKey(User, ...)
    modified_by = models.ForeignKey(User, null=True, blank=True, ...)

When a record is created, we would want to set created_by, but if it’s being updated, we’d want to set modified_by. This use case can’t be solved by using update_or_create, unless it allows for us to specify a different set of default values.

Record.objects.update_or_create(
    some_id=some_value,
    defaults={"modified_by": user},
    create_defaults={"created_by": user},
)

[Timothy Schilling]

Here's a PR with the first version of the changes. ​https://github.com/django/django/pull/16511

[Timothy Schilling]

I ended up going with the create_defaults approach since we have a workaround for the lookup via create_defaults__exact.

[Mariusz Felisiak <felisiak.mariusz@…>]

In c580847:

Fixed #34280 -- Allowed specifying different field values for create operation in QuerySet.update_or_create().