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
24757	Removing unique_together constraint make Django 1.8 migration fail with MySQL	Thomas Recouvreux	Tim Graham	"Migrations containing `migrations.AlterUniqueTogether(name='xxx', unique_together=set([]),)` may fail on Django 1.8 but not on Django 1.7 if using the backend mysql.


Given the database engine is mysql and the following `models.py`:

{{{
from django.db import models


class MyModelA(models.Model):
    s = models.CharField(max_length=120)


class MyModelB(models.Model):
    a = models.ForeignKey(MyModelA)
    b = models.IntegerField()
}}}

And the 2 following migrations:

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

from django.db import models, migrations


class Migration(migrations.Migration):

    dependencies = [
    ]

    operations = [
        migrations.CreateModel(
            name='MyModelA',
            fields=[
                ('id', models.AutoField(serialize=False, auto_created=True, verbose_name='ID', primary_key=True)),
                ('s', models.CharField(max_length=120)),
            ],
        ),
        migrations.CreateModel(
            name='MyModelB',
            fields=[
                ('id', models.AutoField(serialize=False, auto_created=True, verbose_name='ID', primary_key=True)),
                ('b', models.IntegerField()),
                ('a', models.ForeignKey(to='mysite.MyModelA')),
            ],
        ),
        migrations.AlterUniqueTogether(
            name='mymodelb',
            unique_together=set([('a', 'b')]),
        ),
    ]
}}}

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

from django.db import models, migrations


class Migration(migrations.Migration):

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

    operations = [
        migrations.AlterUniqueTogether(
            name='mymodelb',
            unique_together=set([]),
        ),
    ]
}}}


Running `./manage.py migrate` works fine on Django 1.7, but on Django 1.8 I get the following error:

{{{
$ ./manage.py migrate                
Operations to perform:
  Synchronize unmigrated apps: staticfiles, messages
  Apply all migrations: contenttypes, auth, mysite, sessions, admin
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
  Installing custom SQL...
Running migrations:
  Rendering model states... DONE
  [..]
  Applying mysite.0001_initial... OK
  Applying mysite.0002_auto...Traceback (most recent call last):
  [..]
django.db.utils.OperationalError: (1553, ""Cannot drop index 'mysite_mymodelb_a_id_b53c781c93aab9a_uniq': needed in a foreign key constraint"")
}}}


The same migrations work on psql, I did not have the chance to test it against sqlite.

Inspecting the generated sql I found Django 1.7 add an index in addition to the unique constraint and Django 1.8 does not.

On Django 1.7

{{{
BEGIN;
CREATE TABLE `mysite_mymodela` (`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY, `s` varchar(120) NOT NULL);
CREATE TABLE `mysite_mymodelb` (`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY, `b` integer NOT NULL, `a_id` integer NOT NULL);
ALTER TABLE `mysite_mymodelb` ADD CONSTRAINT `mysite_mymodelb_a_id_48275010b87402ed_uniq` UNIQUE (`a_id`, `b`);
ALTER TABLE `mysite_mymodelb` ADD CONSTRAINT `mysite_mymodelb_a_id_4981d3920f0fef1e_fk_mysite_mymodela_id` FOREIGN KEY (`a_id`) REFERENCES `mysite_mymodela` (`id`);
CREATE INDEX `mysite_mymodelb_e2b453f4` ON `mysite_mymodelb` (`a_id`);

COMMIT;
}}}

Note the {{{CREATE INDEX `mysite_mymodelb_e2b453f4` ON `mysite_mymodelb` (`a_id`);}}}.

And on Django 1.8
{{{
BEGIN;
CREATE TABLE `mysite_mymodela` (`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY, `s` varchar(120) NOT NULL);
CREATE TABLE `mysite_mymodelb` (`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY, `b` integer NOT NULL, `a_id` integer NOT NULL);
ALTER TABLE `mysite_mymodelb` ADD CONSTRAINT `mysite_mymodelb_a_id_784bb266d0e363ab_uniq` UNIQUE (`a_id`, `b`);
ALTER TABLE `mysite_mymodelb` ADD CONSTRAINT `mysite_mymodelb_a_id_50892c78e36678b8_fk_mysite_mymodela_id` FOREIGN KEY (`a_id`) REFERENCES `mysite_mymodela` (`id`);

COMMIT;
}}}

I joined the full traceback.
"	Bug	closed	Migrations	1.8	Release blocker	fixed	migrations mysql		Ready for checkin	1	0	0	0	0	0