#23932: models.UUIDField() documentation / migrations
---------------------------------+--------------------
Reporter: michaeljohnbarr | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: master
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
---------------------------------+--------------------
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.
--
Ticket URL: <https://code.djangoproject.com/ticket/23932>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-updates/058.d57deb728e3398e5a9c4eafa9ab1af96%40djangoproject.com.
For more options, visit https://groups.google.com/d/optout.
