Remove UPDATE query when saving a new model instance with a primary key that has a default

[user0007]

Using a model's instance:

class Account(models.Model):
    id = models.UUIDField(
        primary_key=True,
        default=uuid.uuid4,
        editable=False
    )
    title = models.TextField()

>> account = Account()
>> account.title = "abc"
>> account.save()

1. UPDATE "app_account" SET "title" = \'\', WHERE "app_account"."id" = \'67c9327d-150e-419f-b493-0c2c59a045c3\'::uuid',
2. INSERT INTO "app_account" ("title", "id") VALUES (\'abc\', \'3d8c1b3c-214a-4798-a0fa-d4c22c2b877f\'::uuid)

Using a model's manager method:

>> Account.objects.create(title="abc")

1. INSERT INTO "app_account" ("title", "id") VALUES (\'abc\', \'3d8c1b3c-214a-4798-a0fa-d4c22c2b877f\'::uuid)

Using a model's instance with force_insert argument:

>> account = Account()
>> account.title = "abc"
>> account.save(force_insert=true)

1. INSERT INTO "app_account" ("title", "id") VALUES (\'abc\', \'3d8c1b3c-214a-4798-a0fa-d4c22c2b877f\'::uuid)

Related issue? https://code.djangoproject.com/ticket/29129

[Tim Graham]

I'm not sure if the issue can or should be fixed. For the model you gave, an instance will have an id from default=uuid.uuid4, so ​as documented an UPDATE is tried (​code). A fix might try to detect if the primary key came from a default and if so, skip the update.

[Simon Charette]

A fix might try to detect if the primary key came from a default and if so, skip the update.

I think we could use some kind of self._state.adding and self._meta.pk.default heuristics to automatically set force_insert=True on the last table/leaf child. That would break the following scenario though.

a = Account(pk='known-uuid-pk')
a.title = new_title
a.save()  # expects an UPDATE here.

But I would argue that force_update should be passed in this case.

That wouldn't solve the MTI issue described in #29129 but that would do for this case.

[Windson yang]

@Tim Graham, should we still work on it?

[Tim Graham]

Simon's proposal seems fine to me.

[Simon Charette]

Closed #29129 as a duplicate because the root of the issue is really the primary key with a default and not MTI.

[Hasan Ramezani]

PR​https://github.com/django/django/pull/11680

[Mariusz Felisiak <felisiak.mariusz@…>]

In 85458e9:

Fixed #29260 -- Skipped an UPDATE when adding a model instance with primary key that has a default.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 9e14bc2:

Refs #29260 -- Doc'd Model.save() behavior change in Django 3.0.

[Mariusz Felisiak <felisiak.mariusz@…>]

In a04e6fb3:

[3.0.x] Refs #29260 -- Doc'd Model.save() behavior change in Django 3.0.

Backport of 9e14bc2135cb806b66374bac791c79344fff4ded from master