python - Django migration with uuid field generates duplicated values

Question

I have a uuid field (not a primary key). The generated migration is:

from __future__ import unicode_literals

from django.db import migrations, models
import uuid


class Migration(migrations.Migration):

    dependencies = [
        ....
    ]

    operations = [
        ...
        migrations.AddField(
            model_name='device',
            name='uuid',
            field=models.UUIDField(default=uuid.uuid4, unique=True),
        ),
        ...
    ]

But when doing python manage.py migrate it is crashing with:

django.db.utils.IntegrityError: could not create unique index "restaurants_device_uuid_key" DETAIL: Key (uuid)=(f3858ded-b8e0-4ac0-8436-8a61b10efc73) is duplicated.

Strangely enough, the problem does not seem to occur with primary keys (which are maybe created by the database, and not internally by django?)

How can I add a uuid field, and make sure that migrations work?

Answer 1

Here is an example doing everything in one single migration thanks to a RunPython call.

# -*- coding: utf-8 -*
from __future__ import unicode_literals

from django.db import migrations, models
import uuid


def create_uuid(apps, schema_editor):
    Device = apps.get_model('device_app', 'Device')
    for device in Device.objects.all():
        device.uuid = uuid.uuid4()
        device.save()


class Migration(migrations.Migration):

    dependencies = [
        ('device_app', 'XXXX'),
    ]

    operations = [
        migrations.AddField(
            model_name='device',
            name='uuid',
            field=models.UUIDField(blank=True, null=True),
        ),
        migrations.RunPython(create_uuid),
        migrations.AlterField(
            model_name='device',
            name='uuid',
            field=models.UUIDField(unique=True)
        )
    ]