id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
28542	migration that introduces uuid field is not reversible with unique=True	karyon	Tim Martin	"sorry for the confusing title, couldn't think of a better one...

this migration here (from https://github.com/fsr-itse/EvaP/pull/1002/files)

{{{
     # -*- coding: utf-8 -*-
# Generated by Django 1.11.3 on 2017-07-03 18:31
from __future__ import unicode_literals

from django.db import migrations, models
import uuid

def fill_textanswer_uuid(apps, schema_editor):
    db_alias = schema_editor.connection.alias
    TextAnswer = apps.get_model('evaluation', 'TextAnswer')
    for obj in TextAnswer.objects.using(db_alias).all():
        obj.uuid = uuid.uuid4()
        obj.save()

class Migration(migrations.Migration):
    """""" this migration changes a model from a auto-generated id field to a uuid-primary key. """"""

    dependencies = [
        ('evaluation', '0059_add_yes_no_questions'),
    ]

    # Based on
    # https://gist.github.com/smcoll/8bb867dc631433c01fd0

    operations = [
        migrations.AddField(
            model_name='textanswer',
            name='uuid',
            field=models.UUIDField(null=True),
        ),
        migrations.RunPython(fill_textanswer_uuid, migrations.RunPython.noop),
        migrations.AlterField(
            model_name='textanswer',
            name='uuid',
            field=models.UUIDField(primary_key=False, default=uuid.uuid4, serialize=False, editable=False), # add 'unique=True' here
        ),
        migrations.RemoveField('TextAnswer', 'id'),
        migrations.RenameField(
            model_name='textanswer',
            old_name='uuid',
            new_name='id'
        ),
        migrations.AlterField(
            model_name='textanswer',
            name='id',
            field=models.UUIDField(primary_key=True, default=uuid.uuid4, serialize=False, editable=False),
        ),
    ]

}}}

runs fine forwards and backwards.

adding unique=True in the line indicated above makes the backward direction fail, although it should intuitively have little to no effect. especially the error {{{multiple primary keys for table ""evaluation_textanswer"" are not allowed}}} doesn't make much sense to me. traceback:

{{{
Traceback (most recent call last):
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/backends/utils.py"", line 65, in execute
    return self.cursor.execute(sql, params)
psycopg2.ProgrammingError: multiple primary keys for table ""evaluation_textanswer"" are not allowed
 
 
The above exception was the direct cause of the following exception:
 
Traceback (most recent call last):
  File ""./manage.py"", line 10, in <module>
    execute_from_command_line(sys.argv)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/core/management/__init__.py"", line 363, in execute_from_command_line
    utility.execute()
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/core/management/__init__.py"", line 355, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/core/management/base.py"", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/core/management/base.py"", line 330, in execute
    output = self.handle(*args, **options)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/core/management/commands/migrate.py"", line 204, in handle
    fake_initial=fake_initial,
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/migrations/executor.py"", line 119, in migrate
    state = self._migrate_all_backwards(plan, full_plan, fake=fake)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/migrations/executor.py"", line 194, in _migrate_all_backwards
    self.unapply_migration(states[migration], migration, fake=fake)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/migrations/executor.py"", line 264, in unapply_migration
    state = migration.unapply(state, schema_editor)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/migrations/migration.py"", line 178, in unapply
    operation.database_backwards(self.app_label, schema_editor, from_state, to_state)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/migrations/operations/fields.py"", line 160, in database_backwards
    schema_editor.add_field(from_model, to_model._meta.get_field(self.name))
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/backends/base/schema.py"", line 429, in add_field
    self.execute(sql, params)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/backends/base/schema.py"", line 120, in execute
    cursor.execute(sql, params)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/backends/utils.py"", line 80, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/backends/utils.py"", line 65, in execute
    return self.cursor.execute(sql, params)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/utils.py"", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/utils/six.py"", line 685, in reraise
    raise value.with_traceback(tb)
  File ""/home/vagrant/.local/lib/python3.4/site-packages/django/db/backends/utils.py"", line 65, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: multiple primary keys for table ""evaluation_textanswer"" are not allowed
}}}

console output including sql statements when running the migration backwards without the unique=True: https://pastebin.com/gs2SaBjt
with unique=True: https://pastebin.com/QUebX0aa

check them out, the sql is vastly different. i haven't investigated further what the problem is."	Bug	closed	Migrations	1.11	Normal	fixed		Tim Martin	Accepted	1	0	0	0	0	0