adding unique=True to a field that formerly had db_index=True, autodetection fails

[carl@…]

Create a model like this (in myapp/models.py):

class TestModel(models.Model):
    slug = models.SlugField()

Perform its initial migration. Then add unique=True:

class TestModel(models.Model):
    slug = models.SlugField(unique=True)

Auto-create a migration:

$ ./manage.py startmigration myapp --auto unique_slug

And run the migration:

$ ./manage.py migrate myapp
Running migrations for myapp:
 - Migrating forwards to 0002_unique_slug.
 > myapp: 0002_unique_slug
Traceback (most recent call last):
  File "/home/carljm/projects/testproj_env/bin/django-admin.py", line 7, in <module>
    execfile(__file__)
  File "/home/carljm/projects/testproj_env/src/django/django/bin/django-admin.py", line 5, in <module>
    management.execute_from_command_line()
  File "/home/carljm/projects/testproj_env/src/django/django/core/management/__init__.py", line 353, in execute_from_command_line
    utility.execute()
  File "/home/carljm/projects/testproj_env/src/django/django/core/management/__init__.py", line 303, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/carljm/projects/testproj_env/src/django/django/core/management/base.py", line 195, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/home/carljm/projects/testproj_env/src/django/django/core/management/base.py", line 222, in execute
    output = self.handle(*args, **options)
  File "/home/carljm/projects/testproj_env/src/south/south/management/commands/migrate.py", line 91, in handle
    skip = skip,
  File "/home/carljm/projects/testproj_env/src/south/south/migration.py", line 581, in migrate_app
    result = run_forwards(mapp, [mname], fake=fake, db_dry_run=db_dry_run, verbosity=verbosity)
  File "/home/carljm/projects/testproj_env/src/south/south/migration.py", line 388, in run_forwards
    verbosity = verbosity,
  File "/home/carljm/projects/testproj_env/src/south/south/migration.py", line 329, in run_migrations
    runfunc(orm)
  File "/home/carljm/projects/testproj/content/migrations/0002_unique_slug.py", line 11, in forwards
    db.create_unique('myapp_testmodel', ['slug'])
  File "/home/carljm/projects/testproj_env/src/south/south/db/generic.py", line 358, in create_unique
    self.execute("ALTER TABLE %s ADD CONSTRAINT %s UNIQUE (%s)" % (qn(table_name), qn(name), cols))
  File "/home/carljm/projects/testproj_env/src/south/south/db/generic.py", line 82, in execute
    cursor.execute(sql, params)
  File "/home/carljm/projects/testproj_env/src/django/django/db/backends/util.py", line 19, in execute
    return self.cursor.execute(sql, params)
psycopg2.ProgrammingError: relation "myapp_testmodel_slug" already exists

The problem is that an index myapp_testmodel_slug already existed for the db_index=True (which SlugFields? have on by default), and the generated migration tries to create a new unique index with the same name, without deleting the old one.

The manual fix is to add

db.delete_index('myapp_testmodel', ['slug'])

to the beginning of the migration's "forwards" method (before the new unique index is created), and likewise

db.create_index('myapp_testmodel', ['slug'])

to the end of the "backwards" method. But it would be nice if South figured this out itself.

(The problem is the same if the field is not a SlugField? and db_index=True is explicitly specified in the first version of the model.)

[andrew]

Yes, this is the old problem about indexes, uniques, MySQL, and various other things like that. I'm scheduling this for 0.7 to fix it, since there's thankfully a workaround.

[carl@…]

Just a note since you mentioned MySQL: I originally had this problem with Postgres (note psycopg2 in the traceback above), and I just now experienced it with MySQL, so it's at least an issue on those two DB backends, if not others as well.

[andrew]

This appears to be fixed in 0.7.