#25648: Unable to modify or delete unique_together Model constraint (PostgreSQL)
-------------------------------+--------------------
Reporter: MasterKale | Owner: nobody
Type: Uncategorized | Status: new
Component: Uncategorized | Version: 1.8
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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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...
--
Ticket URL: <https://code.djangoproject.com/ticket/25648>
Django <https://code.djangoproject.com/>
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