Add makemigrations warning for unique fields with callable defaults

[Harper04]

Callables on properties for ModelFields are used for various reasons. One use case is to autocreate random file names or user passwords if not present.
The migration seems to call them only once because after the migration every "Buchung" has the same wlan_password.

My Model:

def random_wlan_key():
    return ''.join(random.SystemRandom().choice("1234567890abcdefghkmnpqrstuvwxyz") for i in range(9))

class Buchung(models.Model):
    [...]
    wlan_passwort = models.CharField(max_length=10, default=random_wlan_key)

The generated migration:

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

from django.db import models, migrations
import main.models


class Migration(migrations.Migration):

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

    operations = [
        migrations.AddField(
            model_name='buchung',
            name='wlan_passwort',
            field=models.CharField(default=main.models.random_wlan_key, max_length=10),
            preserve_default=True,
        ),
    ]

[Baptiste Mispelon]

Hi,

I can reproduce this indeed. At this point, I'm not sure if this behavior is by design or not.

If not, we should definitely fix this and if it is, we should document it.

Thanks.

[Harper04]

Hi,

thanks for your time.

I would argue that this behavior is not intended as best practice to create fields like "created_at" fields is to assign a callable.
Calling it only once during a migration is killing dynamic behavior. What if the default value of this field is dependent on another one?

regards

Tom

[Markus Holtermann]

I wouldn't consider this a bug as the default value is nothing that is enforced on database level but inside Django. If you want to have random values per row, you need to add a ​RunPython operation that updates the value per row.

[Andrew Godwin]

This is exactly as designed; when creating columns databases only take a single argument for the DEFAULT clause, so we can only provide a single value, so we only call a callable once. If you want unique values per row, you'll need to populate them in RunPython as Markush2010 suggests.

[Stavros Korokithakis]

I do indeed want unique values per row, and will populate them myself, but, in that case, I don't want Django setting the default and having me wonder down the line why the rows for which I didn't specify a value have a default of 2014-06-24. The correct behavior there would be to ignore the default completely, since there's no reasonable use case where it's supposed to call the callable once and use it for all the columns.

[Michael Barr]

Callables on properties for ModelFields are used for various reasons. One use case is to autocreate UUIDs (​https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.UUIDField).

My Models:

import uuid

class AbstractUUIDModel(models.Model):
    [...]
    uuid = models.UUIDField(default=uuid.uuid4, editable=False, max_length=32, help_text='Universally unique identifier', unique=True, verbose_name='UUID')

    class Meta:
        abstract = True

class Company(AbstractUUIDModel):
    pass

The generated migration:

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

from django.db import models, migrations
import main.models
import uuid


class Migration(migrations.Migration):

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

    operations = [
        migrations.AddField(
            model_name='company',
            name='uuid',
            field=models.UUIDField(default=uuid.uuid4, editable=False, max_length=32, help_text='Universally unique identifier', unique=True, verbose_name='UUID'),
            preserve_default=True,
        ),
    ]

Results:

django.db.utils.IntegrityError: could not create unique index "company_company_uuid_key"
DETAIL:  Key (uuid)=(1ba0e330-9786-4928-b89d-b1fbef72b9c1) is duplicated.

Based on the ​documentation, you should add the default=uuid.uuid4 on the models.UUIDField. However, you cannot do this with the current way migrations work. Django currently tries to generate the same UUID for each instance that is created. In this instance (Django 1.8 isn't released yet), you will run into issues:

import uuid
from django.db import models

class MyUUIDModel(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    # other fields

If default is set to a callable, I believe that it is implied that the result should be dynamically generated in Python/Django. If the functionality is to stay as stated in the result of this ticket, I would have to hand-code this migration for each app that previously existed to first create a nullable UUIDField with no default, then create a data migration to add distinct UUIDs to the Company, then write a migrations.AlterField migration to set null=False. The alternative might be to consider creating a special case for the UUIDField to allow for non-duplicated UUIDs to be created during migrations.

From what I can take away, it has been stated that you cannot add/alter a field with a default=callable(), null=False, unique=True without writing a custom migration. I would be much easier for if migrations followed the same standards as the ​django.db.models.fields.Field.get_default() method. Currently, we have to hand-code the following migration to avoid the failure for every app which utilizes the AbstractUUIDModel, which I feel violates the DRY principle:

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

from django.db import models, migrations
import uuid

def generate_uuid(apps, schema_editor):
    Company = apps.get_model('company', 'Company')
    for company in Company.objects.all().iterator():
        company.uuid = uuid.uuid4()

        company.save()

class Migration(migrations.Migration):

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

    operations = [
        migrations.AddField(
            model_name='company',
            name='uuid',
            field=models.UUIDField(editable=False, max_length=32, help_text='Universally unique identifier', null=True, verbose_name='UUID'),
            preserve_default=False,
        ),
        migrations.RunPython(
            generate_uuid,
        ),
        migrations.AlterField(
            model_name='company',
            name='uuid',
            field=models.UUIDField(editable=False, max_length=32, help_text='Universally unique identifier', unique=True, verbose_name='UUID'),
            preserve_default=True,
        )
    ]

[Danilo Bargen]

I just ran into the same issue (trying to set default UUIDs on an UUID field). Using per-row callables as default value in migrations would be very useful, although it should probably be specified in an explicit way.

[Rowan Seymour]

I also ran into this problem with a UUID field. I would have thought that this is a sufficiently common use case to be catered for in core. But core devs feel that the onus is on people to write custom 3-step migrations each time, then makemigrations should at least show a warning. I'd imagine that in most cases, a single value for all rows is not what the user is expecting. If makemigrations sees unique=True on the field then it knows that the migration will fail.

[Luke Plant]

It seems reasonable to me that makemigrations should, at the very least, issue a warning when it finds a field with a default which is callable and unique==True, and probably for all callable defaults when the field is being added. I'm re-opening on that basis, hope that's OK with other devs.

I think an ideal solution is that the warning should reference some shortcut/info for implementing the needed migration.

[Tim Graham]

The warning could point to this documentation: ​https://docs.djangoproject.com/en/dev/howto/writing-migrations/#migrations-that-add-unique-fields

[max13fr]

Hello,

I'm still running this issue on Django five years later, is there any hope to see this issue fixed ?

I don't understand why default id with primary key (so is unique too) doesn't have the same issue.

Default values are normally used only when we are not providing a value (or providing None value), so I don't understand why when I specify a value, Django just uses it instead of replace it with default value.

What I'm missing ?

If needed here my example:

api/models.py :

class Model Test(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    name = models.CharField(max_length=100)

api/fixtures/initial_data.json :

{
    "model": "api.Test",
    "pk": "724ea599-10a2-49a1-be2f-c1fcb04d003e",
    "fields": {
        "name": "Test 1"
    }
}, {
    "model": "api.Test",
    "pk": "6bcb72f0-099b-416d-8b2c-fcd0bef76cbf",
    "fields": {
        "name": "Test 2"
    }
}

python manage.py loaddata initial_data

django.db.utils.IntegrityError: Problem installing fixture 'api/fixtures/initial_data.json': Could not load api.Test(pk=724ea599-10a2-49a1-be2f-c1fcb04d003e): UNIQUE constraint failed: api_test.id

Thanks in advance
Max

---

EDIT: oh my bad it's not the same issue (here it's about makemigrations / migrate instead of loaddata fixture in my case).
Create a dedicated issue here : https://code.djangoproject.com/ticket/31531#ticket

[Jacob Walls]

​PR

[Jacob Walls]

Marking PNI until I can move this over the to the interactive questioner per Simon's review.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 47f791f:

Fixed #23408 -- Added migrations questioner prompt for adding unique fields with a callable default.