[Django] #36401: Adding CompositePrimaryKey to exiting table leads to drop existing PK with not composit PK creation

[Django]

#36401: Adding CompositePrimaryKey to exiting table leads to drop existing PK with
not composit PK creation
--------------------------------+--------------------------------------
Reporter: Pavel Tyslacki | Type: Bug
Status: new | Component: Migrations
Version: 5.2 | Severity: Normal
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
--------------------------------+--------------------------------------
When I define new table with `CompositePrimaryKey`:

{{{
class Test1(models.Model):
pk = models.CompositePrimaryKey("product_id", "order_id")
product_id = models.BigIntegerField()
order_id = models.BigIntegerField()
quantity = models.IntegerField()
}}}

if creates table with

{{{
migrations.CreateModel(
name='Test1',
fields=[
('pk', models.CompositePrimaryKey('product_id',
'order_id', blank=True, editable=False, primary_key=True,
serialize=False)),
('product_id', models.BigIntegerField()),
('order_id', models.BigIntegerField()),
('quantity', models.IntegerField()),
],
),
}}}

and final SQL:

{{{
CREATE TABLE "blog_test1" ("product_id" bigint NOT NULL, "order_id" bigint
NOT NULL, "quantity" integer NOT NULL, PRIMARY KEY ("product_id",
"order_id"));
}}}

But if I has existing table:

{{{
class Test2(models.Model):
#pk = models.CompositePrimaryKey("product_id", "order_id")
product_id = models.BigIntegerField()
order_id = models.BigIntegerField()
quantity = models.IntegerField()
}}}

{{{
migrations.CreateModel(
name='Test2',
fields=[
('id', models.BigAutoField(auto_created=True,
primary_key=True, serialize=False, verbose_name='ID')),
('product_id', models.BigIntegerField()),
('order_id', models.BigIntegerField()),
('quantity', models.IntegerField()),
],
),
}}}

and try to add `CompositePrimaryKey`, in this case next migration created:


{{{
migrations.RemoveField(
model_name='test2',
name='id',
),
migrations.AddField(
model_name='test2',
name='pk',
field=models.CompositePrimaryKey('product_id', 'order_id',
blank=True, editable=False, primary_key=True, serialize=False),
),
}}}

and migration SQL will look next:

{{{
--
-- Remove field id from test2
--
ALTER TABLE "blog_test2" DROP COLUMN "id" CASCADE;
--
-- Add field pk to test2
--
-- (no-op)
}}}

in this case old PK will be dropped, but new CompositePrimaryKey will not
created.

Expected no matter is model created in one or two steps, final db table
will have same PK constraints, eg. CompositePrimaryKey will be created if
we add it to existing table.

Issue presented at least in postgres and sqlite
--
Ticket URL: <https://code.djangoproject.com/ticket/36401>
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