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
30431	Migration hangs on non default database.	mandm	nobody	"I'm trying to add a UUIDField in an existing user model following this method (with unique=True & default=uuid.uuid4). https://docs.djangoproject.com/en/1.11/howto/writing-migrations/#migrations-that-add-unique-fields

This works with a single database but hangs in the following scenarios with multiple databases : 

- Hangs
  - python manage.py migrate --database=nondefault
  - also hangs with `.using('nondefault')` in the query in gen_uuid function

- Works
  - python manage.py migrate 
  - python manage.py migrate --database=default

It seems to always hang at this statement `UserModel.objects.all()` when a non-default database is used either via `--database` option in migrate or via `.using('nondefault')` in query.

Here's a snippet of the code I've used. I would be glad to provide additional information to reproduce the issue. 

User model : 

{{{
class CustomUser(AbstractUser):
    address = models.TextField()
    uniq_id = models.UUIDField(unique=True, default=uuid.uuid4) 

    class Meta:        
        db_table = 'customuser'
}}}


Migration file


{{{
def gen_uuid(apps, schema_editor):
    UserModel = apps.get_model('myapp', 'customuser')
    for row in UserModel.objects.all():     # .objects.using('nondefault').all(): -- HANGS ! 
        row.uniq_id = uuid.uuid4()
        row.save(update_fields=['uniq_id']) 

class Migration(migrations.Migration):
    dependencies = [
        ('myapp', '003_previous_migration'),
    ]

    operations = [
        migrations.AddField(
            model_name='customuser',
            name='uniq_id',
            field=models.UUIDField(default=uuid.uuid4, null=True),
        ),
        migrations.RunPython(gen_uuid, reverse_code=migrations.RunPython.noop),
        migrations.AlterField(
            model_name='customuser',
            name='uniq_id',
            field=models.UUIDField(default=uuid.uuid4, unique=True),
        ),
    ]
}}}
"	Bug	closed	Migrations	dev	Normal	worksforme	migrations, uuid, uuidfield, database router, multiple database		Unreviewed	0	0	0	0	0	0