Cannot Alter order_with_respect_to after there is already data in a table

[James Pulec]

Due to the way that migration operation 'AlterOrderWithRespectTo' is implemented, it will not set a default for the '_order' column it tries to create. Because of this, it is not possible to change 'order_with_respect_to' once there is already data in your database, since the SQL it generates is 'ALTER TABLE "test_app_eggs" ADD COLUMN "_order" integer NOT NULL;'.

Inital models:

from django.db import models

# Create your models here.


class Spam(models.Model):
    name = models.CharField(max_length=200)


class Eggs(models.Model):
    spam = models.ForeignKey('test_app.Spam')
    name = models.CharField(max_length=200)

Then add some data, and change the models to:

from django.db import models

# Create your models here.


class Spam(models.Model):
    name = models.CharField(max_length=200)


class Eggs(models.Model):
    spam = models.ForeignKey('test_app.Spam')
    name = models.CharField(max_length=200)

    class Meta:
        order_with_respect_to = 'spam'

and then make migrations and try to migrate results in the following traceback:

(test_project)~/test_project/test_project: $ python manage.py migrate
Operations to perform:
  Apply all migrations: admin, test_app, contenttypes, auth, sessions
Running migrations:
  Applying test_app.0002_auto_20141211_2202...Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
    utility.execute()
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 377, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/core/management/base.py", line 338, in execute
    output = self.handle(*args, **options)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/core/management/commands/migrate.py", line 160, in handle
    executor.migrate(targets, plan, fake=options.get("fake", False))
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/migrations/executor.py", line 63, in migrate
    self.apply_migration(migration, fake=fake)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/migrations/executor.py", line 97, in apply_migration
    migration.apply(project_state, schema_editor)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/migrations/migration.py", line 107, in apply
    operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/migrations/operations/models.py", line 328, in database_forwards
    field,
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/backends/schema.py", line 390, in add_field
    self.execute(sql, params)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/backends/schema.py", line 99, in execute
    cursor.execute(sql, params)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/backends/utils.py", line 81, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/james/.virtualenvs/test_project/local/lib/python2.7/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
django.db.utils.IntegrityError: column "_order" contains null values

[Andriy Sokolovskiy]

I'll try to fix that

[Andriy Sokolovskiy]

South sets 0 as default value for new created _order column entries, i think in that case it should be the same.
Or, maybe it needed a new questioner? But it seems to will have no profit.

[Tim Graham]

Yes, I don't think we need a modification to the questioner.

[Andriy Sokolovskiy]

​https://github.com/django/django/pull/3735

[Markus Holtermann]

I think we should set an initial ordering (​https://github.com/django/django/blob/aa5ef0d4fc67a95ac2a5103810d0c87d8c547bac/django/db/models/base.py#L1624-L1633) . Otherwise upcoming saves will start to be ordered, while old data isn't. The ordering of the existing data should be done by the default ordering of the referenced model (model._meta.ordering)

[Tim Graham <timograham@…>]

In 3dbbb8a89ca4beaabd5359fe82e32ed633b15140:

Fixed #23983 -- Fixed a crash in migrations when adding order_with_respect_to to non-empty table.

[Tim Graham <timograham@…>]

In 10482faf1996e654cb9849d7c0065841ad2adf35:

[1.7.x] Fixed #23983 -- Fixed a crash in migrations when adding order_with_respect_to to non-empty table.

Backport of 3dbbb8a89ca4beaabd5359fe82e32ed633b15140 from master