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
25648	Unable to modify or delete unique_together Model constraint (PostgreSQL)	Matthew Miller	nobody	"I'm having the worst luck making any changes to a '''unique_together''' constraint on one of my Models. I want to preface this with the fact that this seems very similar to ticket #23614.

Here's how my model started out:

{{{
class Skill(models.Model):
    company = models.IntegerField(default=1)
    title = models.CharField(max_length=50)

    class Meta:
        unique_together = ['company', 'title']
}}}

I added a '''location''' ForeignKey field to the model, with ''null=True'' set so that I could create a custom migration afterwards to populate the new field. After creating the custom migration, I then ran ''makemigration'' after 1) removing the ''null=True'' constraint on the '''location''' field, 2) altering '''unique_together''' to swap out '''location''' for '''company''', and 3) deleting the '''company''' field.

This is what I ended up with (and remains the ideal state of this Model):

{{{
class Skill(models.Model):
    location = models.ForeignKey(Location, related_name=""skills"")
    title = models.CharField(max_length=50)

    class Meta:
        unique_together = ['location', 'title']
}}}

And this is the migration that Django generated after I made all three changes to Skill:

{{{
    operations = [
        migrations.AlterField(
            model_name='skill',
            name='location',
            field=models.ForeignKey(related_name='skills', to='serverapp.Location'),
        ),
        migrations.AlterUniqueTogether(
            name='skill',
            unique_together=set([('location', 'title')]),
        ),
        migrations.RemoveField(
            model_name='skill',
            name='company',
        ),
    ]
}}}

This migration continually error'd out until I removed the AlterUniqueTogether migration, at which point it ran fine.

Unfortunately, this left me in a bad spot as I could no longer make any changes to '''unique_together''' because (paraphrasing a bit) ""the '''company''' field no longer existed"".

In an effort to rollback things somewhat, I added a '''company''' field back into the Skill Model as a simple IntegerField with ''default=1''. I hoped that I could trick the migration system into a state in which it could modify '''unique_together''' to the desired relationship. This migration processed successfully, so now I had the following Model:

{{{
class Skill(models.Model):
    company = models.IntegerField(default=1)
    location = models.ForeignKey(Location, related_name=""skills"")
    title = models.CharField(max_length=50)

    class Meta:
        unique_together = ['location', 'title']
}}}

Since modifying the constraint kept failing to go through, I then decided to do away with the constraint altogether:

{{{
class Skill(models.Model):
    company = models.IntegerField(default=1)
    location = models.ForeignKey(Location, related_name=""skills"")
    title = models.CharField(max_length=50)

    class Meta:
        # unique_together = ['location', 'title']
}}}

This generated the following expected migration file:

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

This is where I'm currently stuck. Attemping to migrate this change generates the following error:

{{{
Operations to perform:
  Synchronize unmigrated apps: rest_framework_swagger, rest_framework, corsheaders, messages, staticfiles
  Apply all migrations: authtoken, sessions, contenttypes, reversion, serverapp, auth, admin
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
  Installing custom SQL...
Running migrations:
  Rendering model states... DONE
  Applying serverapp.0059_auto_20151030_1823...Traceback (most recent call last):
  File ""shiftserver\manage.py"", line 10, in <module>
    execute_from_command_line(sys.argv)
  File ""C:\Python34\lib\site-packages\django\core\management\__init__.py"", line 338, in execute_from_command_line
    utility.execute()
  File ""C:\Python34\lib\site-packages\django\core\management\__init__.py"", line 330, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ""C:\Python34\lib\site-packages\django\core\management\base.py"", line 390, in run_from_argv
    self.execute(*args, **cmd_options)
  File ""C:\Python34\lib\site-packages\django\core\management\base.py"", line 441, in execute
    output = self.handle(*args, **options)
  File ""C:\Python34\lib\site-packages\django\core\management\commands\migrate.py"", line 221, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File ""C:\Python34\lib\site-packages\django\db\migrations\executor.py"", line 110, in migrate
    self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)
  File ""C:\Python34\lib\site-packages\django\db\migrations\executor.py"", line 147, in apply_migration
    state = migration.apply(state, schema_editor)
  File ""C:\Python34\lib\site-packages\django\db\migrations\migration.py"", line 115, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File ""C:\Python34\lib\site-packages\django\db\migrations\operations\models.py"", line 355, in database_forwards
    getattr(new_model._meta, self.option_name, set()),
  File ""C:\Python34\lib\site-packages\django\db\backends\base\schema.py"", line 322, in alter_unique_together
    "", "".join(columns),
ValueError: Found wrong number (0) of constraints for serverapp_skill(company, title)
}}}

I peeked at schema.py:322 and while it looks as though '''alter_unique_together()''' isn't designed to handle being passed in 0 parameters for unique_together, I'm not confident enough in my reading of '''self._constraint_names()''' to say that the issue exists somewhere in there.

Any ideas what might be going on? I googled the hell out of this issue but couldn't find anything beyond the ticket I mentioned above, and even that's specific to MySQL so it didn't seem to be of much help.

I've lost a couple of hours to this problem. I really hope it's not something stupid I'm trying to do..."	Uncategorized	closed	Uncategorized	1.8	Normal	invalid			Unreviewed	0	0	0	0	0	0