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
28697	Removing unique_together constraint make migration fail with MySQL	Esteban C Borsani	nobody	"I have found Django 1.8.18 fails when removing a model containing a `unique_together` constraint. Django 1.11 works fine. I'm running Python 3.6, mysql 5.7 and using the `mysqlclient` connector.

These are actually two errors:

* When removing just the constraint, the error is `ValueError: Found wrong number (0) of constraints for myapp_topicpollvote(user_id, choice_id)`
* When removing everything altogether the error is `django.db.utils.OperationalError: (1553, ""Cannot drop index 'myapp_topicpollch_poll_id_5bee0f72ec1f3a69_fk_myapp_topicpoll_id': needed in a foreign key constraint"")`

Here is the model

{{{
# -*- coding: utf-8 -*-

from django.db import models
from django.conf import settings


class TopicPoll(models.Model):
    """"""""""""


class TopicPollChoice(models.Model):

    poll = models.ForeignKey(TopicPoll, on_delete=models.CASCADE)


class TopicPollVote(models.Model):

    choice = models.ForeignKey(TopicPollChoice, on_delete=models.CASCADE)
    user = models.ForeignKey(settings.AUTH_USER_MODEL)

    class Meta:
        # removing this raises:
        # ValueError: Found wrong number (0) of constraints for myapp_topicpollvote(user_id, choice_id)

        # removing all models raises
        # django.db.utils.OperationalError: (1553, ""Cannot drop index ...
        unique_together = (('user', 'choice'),)

}}}

Initial migration:

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

from django.db import migrations, models
from django.conf import settings


class Migration(migrations.Migration):

    dependencies = [
        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
    ]

    operations = [
        migrations.CreateModel(
            name='TopicPoll',
            fields=[
                ('id', models.AutoField(verbose_name='ID', primary_key=True, serialize=False, auto_created=True)),
            ],
        ),
        migrations.CreateModel(
            name='TopicPollChoice',
            fields=[
                ('id', models.AutoField(verbose_name='ID', primary_key=True, serialize=False, auto_created=True)),
                ('poll', models.ForeignKey(to='myapp.TopicPoll')),
            ],
        ),
        migrations.CreateModel(
            name='TopicPollVote',
            fields=[
                ('id', models.AutoField(verbose_name='ID', primary_key=True, serialize=False, auto_created=True)),
                ('choice', models.ForeignKey(to='myapp.TopicPollChoice')),
                ('user', models.ForeignKey(to=settings.AUTH_USER_MODEL)),
            ],
        ),
        migrations.AlterUniqueTogether(
            name='topicpollvote',
            unique_together=set([('user', 'choice')]),
        ),
    ]

}}}

Removing just the unique_together:

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

from django.db import migrations, models


class Migration(migrations.Migration):

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

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

}}}

Removing everything altogether (unique_together + all models):

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

from django.db import migrations, models


class Migration(migrations.Migration):

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

    operations = [
        migrations.RemoveField(
            model_name='topicpollchoice',
            name='poll',
        ),
        migrations.AlterUniqueTogether(
            name='topicpollvote',
            unique_together=set([]),
        ),
        migrations.RemoveField(
            model_name='topicpollvote',
            name='choice',
        ),
        migrations.RemoveField(
            model_name='topicpollvote',
            name='user',
        ),
        migrations.DeleteModel(
            name='TopicPoll',
        ),
        migrations.DeleteModel(
            name='TopicPollChoice',
        ),
        migrations.DeleteModel(
            name='TopicPollVote',
        ),
    ]

}}}"	Bug	new	Migrations	1.8	Normal		migrations		Unreviewed	0	0	0	0	0	0