[FIXED] Problem with Updating unique together constraint Django

Issue

Tried to updating unqiue_together constraint using Django-ORM

While Migrating getting following error :

File "/Applications/PyCharm CE.app/Contents/helpers/pydev/pydevd.py", line 1664, in <module>
main()
File "/Applications/PyCharm CE.app/Contents/helpers/pydev/pydevd.py", line 1658, in main
globals = debugger.run(setup['file'], None, None, is_module)
File "/Applications/PyCharm CE.app/Contents/helpers/pydev/pydevd.py", line 1068, in run
pydev_imports.execfile(file, globals, locals)  # execute the script
File "/Applications/PyCharm CE.app/Contents/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
exec(compile(contents+"\n", file, 'exec'), glob, loc)
File "/Users/arpitkoolwal/DEV/Gateway-26Oct/gateway/gateway/manage.py", line 24, in <module>
execute_from_command_line(sys.argv)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
utility.execute()
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/core/management/__init__.py", line 356, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
self.execute(*args, **cmd_options)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
output = self.handle(*args, **options)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 204, in handle
fake_initial=fake_initial,
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/db/migrations/executor.py", line 115, in migrate
state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/db/migrations/executor.py", line 145, in _migrate_all_forwards
state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
state = migration.apply(state, schema_editor)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/db/migrations/migration.py", line 129, in apply
operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/db/migrations/operations/models.py", line 536, in database_forwards
getattr(new_model._meta, self.option_name, set()),
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 365, in alter_unique_together
self._delete_composed_index(model, fields, {'unique': True}, self.sql_delete_unique)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/db/backends/mysql/schema.py", line 88, in _delete_composed_index
return super(DatabaseSchemaEditor, self)._delete_composed_index(model, fields, *args)
File "/Users/arpitkoolwal/DEV/cld/pyenv3/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 394, in _delete_composed_index
", ".join(columns), ValueError: Found wrong number (0) of constraints for test(a, b)

Django : 1.11.15

Error Ticket : Django-Offical

Can anyone tell me, How can we modify existing unique constraint using Django-ORM ?

EDIT :

Model -

class test(models.Model):
    a = models.CharField(max_length=500, blank=True, null=True)
    b = models.CharField(max_length=500, blank=True, null=True)
    c = models.CharField(max_length=500, blank=True, null=True)
    d = models.CharField(max_length=500, blank=True, null=True)
    class Meta:
     unique_together = ('a', 'b')

Migration Code :

class Migration(migrations.Migration):

dependencies = [
    ('demo', '0230_auto_20181106_1243'),
]

operations = [
    migrations.AlterUniqueTogether(
        name='test',
        unique_together=set([a,b]),
    ),
]

Solution

How I solved the problem:

1. python -m pdb manage.py migrate

2. type up to go upper frame. It will go to

   ./python3.6/site-packages/django/db/backends/base/schema.py(363)alter_unique_together() -> self._delete_composed_index(model, fields, {'unique': True}, self.sql_delete_unique)

3. type a to show variables

Notice

old_unique_together = {('a', 'b')}
new_unique_together = {('a', 'b', 'c')}

that the old_unique_together does not exist, according to pg \d tablename

4. Go to old migration files and find old migrations.AlterUniqueTogether, comment them, then run the migration again.
5. ???
6. profit.

Answered By - est