Attempting to recreate the PK in a model with no other fields generates a migration that crashes on SQLite

[Carol Naranjo]

When a model contains no fields other than the primary key, and the PK is recreated (e.g., renaming and switching from an AutoField to a UUIDField), Django generates a migration with two operations: RemoveField followed by AddField.

In SQLite, applying this migration fails because SQLite cannot create a table with no columns. It only affects SQLite because in other databases the table can be altered by droping the column, whereas SQLite requires that a new table is created and then the data is copied.

Steps to reproduce:

1. Starting with a model with only the primary key:

   class Place(models.Model):
       pass # PK is created by default as AutoField with column name "id"
   

2. Modify the primary key:

   class Place(models.Model):
       uuid = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
   

3. Run makemigrations. Which generates following migration:

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0001_initial'),
    ]

    operations = [
        migrations.RemoveField(
            model_name='place',
            name='id',
        ),
        migrations.AddField(
            model_name='place',
            name='uid',
            field=models.AutoField(primary_key=True, serialize=False),
        ),
    ]

4. Apply the migration using SQLite.

Observed behavior:

During the RemoveField operation, the generated migration tries to create a table with zero non-PK columns, resulting in:

CREATE TABLE "new__myapp_place" (); (params None)
    sqlite3.OperationalError: near ")": syntax error

Expected behavior:

SQLite migration should not crash. Either prevent a migration that would create a table with zero columns or handle it gracefully.

On a side note: This is an edge case—tables without additional fields are rare—but it still causes an unexpected crash.

[Jacob Walls]

Thanks, reproduced at f6167b8bc881babd19b67c004e8f37954afc192e. Looks like another circumstance that manifests the failure described in #24424.

[Simon Charette]

Related tickets are #22997 and #29790.

The way SQLite ​requires the table to be rebuilt on field removal and alterations would likely force us to add a _django_empty_col column to support this workflow.

There might be a way to approach this at the auto-detector level by turning a removal and addition of a Field(primary_key=True) with a different name as a [AlterField, RenameField] instead of a [RemoveField, AddField]. Since a model/table can only have one primary key at a time that seems like a better way to approach this problem as it would then allow us to focus our efforts on getting AlterField(from_pk, to_pk) to work in most cases which is what #29790 is about.

[Carol Naranjo]

Replying to Jacob Walls:

Thanks, reproduced at f6167b8bc881babd19b67c004e8f37954afc192e. Looks like another circumstance that manifests the failure described in #24424.

Hi Jacob, can you double check the commit link? it seems unrelated.

---

Replying to Simon Charette:

There might be a way to approach this at the auto-detector level by turning a removal and addition of a Field(primary_key=True) with a different name as a [AlterField, RenameField] instead of a [RemoveField, AddField].

Hi Simon, I like that approach. If it’s not time-sensitive, I’d be happy to work on a fix and assign the ticket to myself.

[Jacob Walls]

Oh it's definitely unrelated. I wasn't blaming that commit, I was just leaving a reference in case it's retriaged a few years from now and we can't reproduce.

If it’s not time-sensitive, I’d be happy to work on a fix and assign the ticket to myself.

Not time-sensitive, thanks for the offer!

[Simon Charette]

Sure thing Carol, please assign the ticket to yourself and take some time to read ​the contribution guidelines.

You'll like be looking at code living in autodetector.py and associated tests in tests/migrations/test_autodetector.py.

[Carol Naranjo]

@Jacob, ah got you, thanks for the clarification :)

Thanks for the hint Simon!

[Carol Naranjo]

Hello Jacob, Simon, I'm back after a short holiday break, happy Easter!

I have now created a PR ready for review here: ​https://github.com/django/django/pull/21073 . Let me know if something is missing. Thanks!

[Carol Naranjo]

By the way, regarding the PR tags, it might be worth mentioning that I’m part of the current Djangonauts cohort

[Simon Charette]

Thanks for the patch Carol, excited to hear you are part of Djangonauts!

I left some comments on the PR. It is heading in the right direction but it should offer the possibility of a rename and not force it when a primary is changed because we won't be able to support all field type alterations so we need an escape hatch to allow the users to drop the field and create a new one when it's the case.

[Carol Naranjo]

hi Simon, sorry for the slow updates (life in between). I updated the PR again according to your remarks, I hope the changes are clear, let me know :)