#27245: can't revert migration with index_together with one field
-------------------------------+--------------------------------------
     Reporter:  rm_            |                    Owner:  nobody
         Type:  Bug            |                   Status:  new
    Component:  Uncategorized  |                  Version:  1.8
     Severity:  Normal         |               Resolution:
     Keywords:                 |             Triage Stage:  Unreviewed
    Has patch:  0              |      Needs documentation:  0
  Needs tests:  0              |  Patch needs improvement:  0
Easy pickings:  0              |                    UI/UX:  0
-------------------------------+--------------------------------------
Changes (by rm_):

 * needs_better_patch:   => 0
 * needs_tests:   => 0
 * needs_docs:   => 0


Old description:

> With django 1.8.14 i added a composite index with one field (lame,
> sorry!) because the field is from an abstract model and it seemed the
> cleanest way to handle that:
>
> {{{
> +    class Meta:
> +        index_together = ['owner']
> }}}
>
> The migrations got created and applied finely:
> {{{
> # -*- coding: utf-8 -*-
> from __future__ import unicode_literals
>
> from django.db import migrations, models
>

> class Migration(migrations.Migration):
>
>     dependencies = [
>         ('foobar', '0002_auto_20160315_1805'),
>     ]
>
>     operations = [
>         migrations.AlterIndexTogether(
>             name='answer',
>             index_together=set([('owner',)]),
>         ),
>     ]
> }}}
>
> Problem is i can't revert the migration:
>
> {{{
>   File "manage.py", line 10, in <module>
>     execute_from_command_line(sys.argv)
>   File "/venv/local/lib/python2.7/site-
> packages/django/core/management/__init__.py", line 354, in
> execute_from_command_line
>     utility.execute()
>   File "/venv/local/lib/python2.7/site-
> packages/django/core/management/__init__.py", line 346, in execute
>     self.fetch_command(subcommand).run_from_argv(self.argv)
>   File "/venv/local/lib/python2.7/site-
> packages/django/core/management/base.py", line 394, in run_from_argv
>     self.execute(*args, **cmd_options)
>   File "/venv/local/lib/python2.7/site-
> packages/django/core/management/base.py", line 445, in execute
>     output = self.handle(*args, **options)
>   File "/venv/local/lib/python2.7/site-
> packages/django/core/management/commands/migrate.py", line 222, in handle
>     executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
>   File "/venv/local/lib/python2.7/site-
> packages/django/db/migrations/executor.py", line 112, in migrate
>     self.unapply_migration(states[migration], migration, fake=fake)
>   File "/venv/local/lib/python2.7/site-
> packages/django/db/migrations/executor.py", line 168, in
> unapply_migration
>     state = migration.unapply(state, schema_editor)
>   File "/venv/local/lib/python2.7/site-
> packages/django/db/migrations/migration.py", line 162, in unapply
>     operation.database_backwards(self.app_label, schema_editor,
> from_state, to_state)
>   File "/venv/local/lib/python2.7/site-
> packages/django/db/migrations/operations/models.py", line 415, in
> database_backwards
>     return self.database_forwards(app_label, schema_editor, from_state,
> to_state)
>   File "/venv/local/lib/python2.7/site-
> packages/django/db/migrations/operations/models.py", line 411, in
> database_forwards
>     getattr(new_model._meta, self.option_name, set()),
>   File "/venv/local/lib/python2.7/site-
> packages/django/db/backends/base/schema.py", line 336, in
> alter_index_together
>     self._delete_composed_index(model, fields, {'index': True},
> self.sql_delete_index)
>   File "venv/local/lib/python2.7/site-
> packages/django/db/backends/base/schema.py", line 349, in
> _delete_composed_index
>     ", ".join(columns),
> ValueError: Found wrong number (2) of constraints for
> cicero_academy_answeracademy(owner_id)
> }}}
>
> So the easy way to fix this is to don't permit te creation of a migration
> with less than two fields. OTOH it would be quite cool to use the non
> composite way of removing an index if the constraints are not enough.
> Whatever you decide i can spend some time on the fix.

New description:

 With django 1.8.14 and postgresql 9.4 i added a composite index with one
 field (lame, sorry!) because the field is from an abstract model and it
 seemed the cleanest way to handle that:

 {{{
 +    class Meta:
 +        index_together = ['owner']
 }}}

 The migrations got created and applied finely:
 {{{
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals

 from django.db import migrations, models


 class Migration(migrations.Migration):

     dependencies = [
         ('foobar', '0002_auto_20160315_1805'),
     ]

     operations = [
         migrations.AlterIndexTogether(
             name='answer',
             index_together=set([('owner',)]),
         ),
     ]
 }}}

 Problem is i can't revert the migration:

 {{{
   File "manage.py", line 10, in <module>
     execute_from_command_line(sys.argv)
   File "/venv/local/lib/python2.7/site-
 packages/django/core/management/__init__.py", line 354, in
 execute_from_command_line
     utility.execute()
   File "/venv/local/lib/python2.7/site-
 packages/django/core/management/__init__.py", line 346, in execute
     self.fetch_command(subcommand).run_from_argv(self.argv)
   File "/venv/local/lib/python2.7/site-
 packages/django/core/management/base.py", line 394, in run_from_argv
     self.execute(*args, **cmd_options)
   File "/venv/local/lib/python2.7/site-
 packages/django/core/management/base.py", line 445, in execute
     output = self.handle(*args, **options)
   File "/venv/local/lib/python2.7/site-
 packages/django/core/management/commands/migrate.py", line 222, in handle
     executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
   File "/venv/local/lib/python2.7/site-
 packages/django/db/migrations/executor.py", line 112, in migrate
     self.unapply_migration(states[migration], migration, fake=fake)
   File "/venv/local/lib/python2.7/site-
 packages/django/db/migrations/executor.py", line 168, in unapply_migration
     state = migration.unapply(state, schema_editor)
   File "/venv/local/lib/python2.7/site-
 packages/django/db/migrations/migration.py", line 162, in unapply
     operation.database_backwards(self.app_label, schema_editor,
 from_state, to_state)
   File "/venv/local/lib/python2.7/site-
 packages/django/db/migrations/operations/models.py", line 415, in
 database_backwards
     return self.database_forwards(app_label, schema_editor, from_state,
 to_state)
   File "/venv/local/lib/python2.7/site-
 packages/django/db/migrations/operations/models.py", line 411, in
 database_forwards
     getattr(new_model._meta, self.option_name, set()),
   File "/venv/local/lib/python2.7/site-
 packages/django/db/backends/base/schema.py", line 336, in
 alter_index_together
     self._delete_composed_index(model, fields, {'index': True},
 self.sql_delete_index)
   File "venv/local/lib/python2.7/site-
 packages/django/db/backends/base/schema.py", line 349, in
 _delete_composed_index
     ", ".join(columns),
 ValueError: Found wrong number (2) of constraints for
 cicero_academy_answeracademy(owner_id)
 }}}

 So the easy way to fix this is to don't permit te creation of a migration
 with less than two fields. OTOH it would be quite cool to use the non
 composite way of removing an index if the constraints are not enough.
 Whatever you decide i can spend some time on the fix.

--

--
Ticket URL: <https://code.djangoproject.com/ticket/27245#comment:1>
Django <https://code.djangoproject.com/>
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