Re: [Django] #36947: Changing a field's db_comment unnecessarily alters column type (was: Changing db_comment / GeneratedField error)

[Django]

#36947: Changing a field's db_comment unnecessarily alters column type
------------------------------+-----------------------------------------
Reporter: Alex Fischer | Owner: Clifford Gama
Type: Bug | Status: assigned
Component: Migrations | Version: 6.0
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
------------------------------+-----------------------------------------
Changes (by Clifford Gama):

* summary: Changing db_comment / GeneratedField error => Changing a
field's db_comment unnecessarily alters column type


Old description:

> Django==5.2.11
> Postgres 16.2
>
> With this model definition:
>
> {{{
> class Foo(models.Model):
> foo = models.IntegerField()
> foo_copy = models.GeneratedField(
> expression=models.F("foo"),
> output_field=models.IntegerField(),
> db_persist=True,
> )
> }}}
>
> makemigrations and migrate run fine.
>
> Now add a db_comment to foo:
> {{{
> class Foo(models.Model):
> foo = models.IntegerField(db_comment="foo")
> foo_copy = models.GeneratedField(
> expression=models.F("foo"),
> output_field=models.IntegerField(),
> db_persist=True,
> )
> }}}
>
> Now if we makemigrations and migrate again, we get the following error:
> {{{
> django.db.utils.NotSupportedError: cannot alter type of a column used by
> a generated column
> DETAIL: Column "foo" is used by generated column "foo_copy".
> }}}
>
> Running sqlmigrate on the migration that was produced, we can see what
> the migration is trying to perform:
> {{{
> BEGIN;
> --
> -- Alter field foo on foo
> --
> ALTER TABLE "my_app_foo" ALTER COLUMN "foo" TYPE integer;
> COMMENT ON COLUMN "issue_tracker_foo"."foo" IS 'foo';
> COMMIT;
> }}}
>
> I don't know why it runs the first ALTER COLUMN statement. The ALTER
> COLUMN statement is a no-op most of the time, and that's fine. But in
> this case, it throws an error in Postgres ("cannot alter type of a column
> used by a generated column").

New description:

(On databases that support altering the comment independently of the
type.)

Django==5.2.11
Postgres 16.2

With this model definition:

{{{
class Foo(models.Model):
foo = models.IntegerField()
foo_copy = models.GeneratedField(
expression=models.F("foo"),
output_field=models.IntegerField(),
db_persist=True,
)
}}}

makemigrations and migrate run fine.

Now add a db_comment to foo:
{{{
class Foo(models.Model):
foo = models.IntegerField(db_comment="foo")
foo_copy = models.GeneratedField(
expression=models.F("foo"),
output_field=models.IntegerField(),
db_persist=True,
)
}}}

Now if we makemigrations and migrate again, we get the following error:
{{{
django.db.utils.NotSupportedError: cannot alter type of a column used by a
generated column
DETAIL: Column "foo" is used by generated column "foo_copy".
}}}

Running sqlmigrate on the migration that was produced, we can see what the
migration is trying to perform:
{{{
BEGIN;
--
-- Alter field foo on foo
--
ALTER TABLE "my_app_foo" ALTER COLUMN "foo" TYPE integer;
COMMENT ON COLUMN "issue_tracker_foo"."foo" IS 'foo';
COMMIT;
}}}

I don't know why it runs the first ALTER COLUMN statement. The ALTER
COLUMN statement is a no-op most of the time, and that's fine. But in this
case, it throws an error in Postgres ("cannot alter type of a column used
by a generated column").

--
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Ticket URL: <https://code.djangoproject.com/ticket/36947#comment:2>
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