id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
34879	"""Data truncated for column .."" for migration changing auto-id-field"	Wolfgang Fehr	nobody	"== Introduction (i.e. how this happened) ==

Came across this with updating my system and upgrading `djangocms_blog` vom version `1.2.3 => 2.0.5`.

The migration `0041` changes the ID-fields of the models from `INT(11)` to `BIGINT(20)`, or `AutoField` to `BigAutoField`.
([https://github.com/nephila/djangocms-blog/blob/develop/djangocms_blog/migrations/0041_auto_20230720_1508.py migration 0041 of djangocms-blog])

Most systems just ran the migration successfully, but some failed.
The failed ones even stated ""OK"" in the migration output, even though the migration itself failed and wasn't applied.
(also very interesting to see that behaviour)

== Resulting error ==

The migration basically failed (but not in all systems) with following error:
{{{
...
django.db.utils.DataError: (1265, ""Data truncated for column 'parent_id' at row 1"")
  Applying djangocms_blog.0041_auto_20230720_1508...%
}}}

See full [attachment:traceback.txt traceback] in attachments.

Since this error didn't occur in every system, I assume this might be a non-deterministic behaviour of some kind.
With this ""ID-change"" 2 database-columns will be changed: `id` and `some_self_foreignkey_id`.
My guess would be that the latter sometimes is changed on second position.
This results in `id` being `BIGINT(20)` and `some_self_foreignkey_id` being `INT(11)`.
With this, the reference fails due to ""referenced column can store more data"".

In my case, the `ForeignKey`-fields were all `NULL`, so the migration doesn't care about the data itself, just the schema.

(I don't know if this problem is `django=3.2`-specific or if it also might occur on newer versions)

In the local environment and with integration-tests, everything works fine.

~~My solution for now - to make sure there isn't a downtime due to migrations - is to execute following SQL before migrating:~~
~~`ALTER TABLE someapp_somemodel CHANGE some_self_foreignkey_id some_self_foreignkey_id BIGINT(20) NULL;`~~
''(edit: that alone doesn't work due to the constraints)''

----
== Environment Information ==
- Database: MySQL 5.7.40
- python: 3.11
- django: 3.2.21
- django-cms: 3.11.4
- djangocms-blog: 2.0.5

== Basic Example of Model + Migration ==

{{{#!python
class SomeModel(models.Model):
    some_self_foreignkey = models.ForeignKey(
        ""self"",
        null=True,
        blank=True,
        related_name=""+"",
        on_delete=models.CASCADE,
    )
}}}

When changing ""Auto-Id-Field"" from `AutoField` to `BigAutoField` via `AppConfig` or `settings.py`, following migration-operation would be done:

{{{#!python
migrations.AlterField(
    model_name=""somemodel"",
    name=""id"",
    field=models.BigAutoField(
        auto_created=True,
        primary_key=True,
        serialize=False,
        verbose_name=""ID"",
    ),
),
}}}"	Bug	closed	Migrations	3.2	Normal	worksforme			Unreviewed	0	0	0	0	0	0