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
23932	Document how to set a unique value for new fields using migrations	Michael Barr	andrei kulakov	"This ticket questions two items: the `models.UUIDField` documentation, and the action of a callable default on a model field for migrations.

The current [https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.UUIDField documentation for a models.UUIDField] states to have a default set to `default=uuid.uuid4`.
Assuming I originally had the following model:
{{{#!python
from django.db import models
class TestUUID(models.Model):
    name = models.CharField(max_length=15)
}}}

Then I run a makemigrations and a migrate to create the model.

I then run:
{{{#!python
from testapp.models import TestUUID
TestUUID.objects.create(name='Test 1')
TestUUID.objects.create(name='Test 2')
TestUUID.objects.create(name='Test 3')
TestUUID.objects.create(name='Test 4')
TestUUID.objects.create(name='Test 5')
}}}

I decide to add a models.UUIDField to the model:
{{{#!python
from django.db import models
import uuid
class TestUUID(models.Model):
    name = models.CharField(max_length=15)
    uuid = models.UUIDField(default=uuid.uuid4, unique=True)
}}}

I run makemigrations, which generates:
{{{#!python
# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models, migrations
import uuid


class Migration(migrations.Migration):

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

    operations = [
        migrations.AddField(
            model_name='testuuid',
            name='uuid',
            field=models.UUIDField(default=uuid.uuid4, unique=True, max_length=32),
            preserve_default=True,
        ),
    ]
}}}

When I run migrate, I get the following error:
{{{
  Applying testapp.0002_testuuid_uuid...Traceback (most recent call last):
  File ""manage.py"", line 10, in <module>
    execute_from_command_line(sys.argv)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/core/management/__init__.py"", line 338, in execute_from_command_line
    utility.execute()
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/core/management/__init__.py"", line 330, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/core/management/base.py"", line 390, in run_from_argv
    self.execute(*args, **cmd_options)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/core/management/base.py"", line 442, in execute
    output = self.handle(*args, **options)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/core/management/commands/migrate.py"", line 193, in handle
    executor.migrate(targets, plan, fake=options.get(""fake"", False))
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/migrations/executor.py"", line 68, in migrate
    self.apply_migration(migration, fake=fake)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/migrations/executor.py"", line 102, in apply_migration
    migration.apply(project_state, schema_editor)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/migrations/migration.py"", line 108, in apply
    operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/migrations/operations/fields.py"", line 51, in database_forwards
    field,
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/backends/sqlite3/schema.py"", line 167, in add_field
    self._remake_table(model, create_fields=[field])
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/backends/sqlite3/schema.py"", line 135, in _remake_table
    self.quote_name(model._meta.db_table),
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/backends/schema.py"", line 100, in execute
    cursor.execute(sql, params)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/backends/utils.py"", line 80, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/backends/utils.py"", line 65, in execute
    return self.cursor.execute(sql, params)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/utils.py"", line 95, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/backends/utils.py"", line 65, in execute
    return self.cursor.execute(sql, params)
  File ""/Users/michaelbarr/.virtualenvs/django_18_venv/src/django/django/db/backends/sqlite3/base.py"", line 518, in execute
    return Database.Cursor.execute(self, query, params)
django.db.utils.IntegrityError: column uuid is not unique
}}}

I have seen the response to [https://code.djangoproject.com/ticket/23408 Automaticly created migration calls callable for default only once], which states:

> 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 ​[https://docs.djangoproject.com/en/1.7/ref/migration-operations/#django.db.migrations.operations.RunPython RunPython operation] that updates the value per row.

> 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.

[https://code.djangoproject.com/ticket/23617 The new UUID field does not allow to automatically generate a UUID on save] also discusses this issue, talking of the possibility of `auto_add` and `auto_add_now`.

It is my personal belief that if a developer sets the model field's `default` value to be a callable, it is meant to be handled by Python, not the database. In my instance above, I am stating that I want a new uuid.uuid4 to be created as the default. This works GREAT for forms and new saves, but not for the migrations since they are all the same `uuid.uuid4()`, as the migrations casts the default value to the called value. From what I can tell, I am not the only person that is confused by the defaults. In Python, the defaults work as expected, but database migrations do not yield the same results.

What I am attempting to do (eventually) is create an abstract model which is reused in multiple applications as a convention. However, this model has existed for quite some time, so each application already has multiple models created. I feel that the `models.UUIDField` should consider correcting the documentation or making a note of it, or reconsider how migration defaults are handled as a whole. I wrote about this as a comment [https://code.djangoproject.com/ticket/23408#comment:6 here] regarding how much work it is to work around this issue. If that is to be the case, so be it, but please be sure to make this clear in the documentation. "	Cleanup/optimization	closed	Documentation	dev	Normal	fixed		info+coding@…	Accepted	1	0	0	1	0	0