Broken squashmigration when model related to another one gets deleted

[Piotr Maliński]

I've got a simple example of an broken squashmigration I had in two apps. The problem is that if a model gets deleted, that has relation to other model then the squash will crash as something will want to do some operation on the non-existing model (relation presence is essential, tested on postgres):

Initial:

class Category(models.Model):
    title = models.CharField(max_length=100)


class News(models.Model):
    title = models.CharField(max_length=100)
    category = models.ForeignKey(Category)

Second migration:

class Category(models.Model):
    title = models.CharField(max_length=100)

class NewCategory(models.Model):
    title = models.CharField(max_length=100)


class News(models.Model):
    title = models.CharField(max_length=100)
    category = models.ForeignKey(Category, blank=True, null=True)
    new_category = models.ForeignKey(NewCategory, blank=True, null=True)

Third:

class NewCategory(models.Model):
    title = models.CharField(max_length=100)


class News(models.Model):
    title = models.CharField(max_length=100)
    new_category = models.ForeignKey(NewCategory, blank=True, null=True)

Squash looks like so:

    operations = [
        migrations.CreateModel(
            name='Category',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('title', models.CharField(max_length=100)),
            ],
        ),
        migrations.CreateModel(
            name='News',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('title', models.CharField(max_length=100)),
                ('category', models.ForeignKey(blank=True, to='relateddeleted.Category', null=True)),
            ],
        ),
        migrations.CreateModel(
            name='NewCategory',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('title', models.CharField(max_length=100)),
            ],
        ),
        migrations.AddField(
            model_name='news',
            name='new_category',
            field=models.ForeignKey(blank=True, to='relateddeleted.NewCategory', null=True),
        ),
        migrations.RemoveField(
            model_name='news',
            name='category',
        ),
        migrations.DeleteModel(
            name='Category',
        ),
    ]

And on a clean database it throws:

  Applying relateddeleted.0001_squashed_0003_auto_20150524_2020...Traceback (most recent call last):
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
psycopg2.ProgrammingError: relation "relateddeleted_category" does not exist


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/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
    utility.execute()
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/__init__.py", line 330, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/commands/test.py", line 30, in run_from_argv
    super(Command, self).run_from_argv(argv)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/base.py", line 390, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/commands/test.py", line 74, in execute
    super(Command, self).execute(*args, **options)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/base.py", line 441, in execute
    output = self.handle(*args, **options)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/commands/test.py", line 90, in handle
    failures = test_runner.run_tests(test_labels)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/test/runner.py", line 210, in run_tests
    old_config = self.setup_databases()
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/test/runner.py", line 166, in setup_databases
    **kwargs
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/test/runner.py", line 370, in setup_databases
    serialize=connection.settings_dict.get("TEST", {}).get("SERIALIZE", True),
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/db/backends/base/creation.py", line 368, in create_test_db
    test_flush=not keepdb,
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/__init__.py", line 120, in call_command
    return command.execute(*args, **defaults)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/base.py", line 441, in execute
    output = self.handle(*args, **options)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/core/management/commands/migrate.py", line 221, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/db/migrations/executor.py", line 110, in migrate
    self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/db/migrations/executor.py", line 147, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 91, in __exit__
    self.execute(sql)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 111, in execute
    cursor.execute(sql, params)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/db/utils.py", line 97, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/utils/six.py", line 658, in reraise
    raise value.with_traceback(tb)
  File "/home/piotr/devel/envs/migracje/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: relation "relateddeleted_category" does not exist

[Tim Graham]

I had to add related_name in step two to avoid a clashing accessor error from the system check error. With that, when I tried to reproduce this, I got the following squashed migration:

operations = [
    migrations.CreateModel(
        name='News',
        fields=[
            ('id', models.AutoField(primary_key=True, serialize=False, auto_created=True, verbose_name='ID')),
            ('title', models.CharField(max_length=100)),
            ('category', models.ForeignKey(blank=True, null=True, to='squash.NewCategory', related_name='foo')),
        ],
    ),
    migrations.CreateModel(
        name='NewCategory',
        fields=[
            ('id', models.AutoField(primary_key=True, serialize=False, auto_created=True, verbose_name='ID')),
            ('title', models.CharField(max_length=100)),
        ],
    ),
    migrations.AddField(
        model_name='news',
        name='new_category',
        field=models.ForeignKey(blank=True, null=True, to='squash.NewCategory'),
    ),
    migrations.RemoveField(
        model_name='news',
        name='category',
    ),
]

This has the problem that NewCategory is created after it's used in the ForeignKey which results in ValueError: Related model 'squash.NewCategory' cannot be resolved when migrating. Accepting at least on that basis.

[Piotr Maliński]

Looks totally different. Old FK gets null/blank and new FK gets blank/null from start, then the old one is dropped. Without the need for related_name.

[Piotr Maliński]

Here is my example: ​https://github.com/riklaunim/django-examples/tree/master/suqashexamples/relateddeleted/migrations

[Tim Graham]

There was a typo in the "Second migration" step in the description: category = models.ForeignKey(NewCategory, ... instead of ForeignKey(Category .... I fixed that and can now reproduce the same traceback you reported as of a76309f007fff6957f39190b06f8a9055f73130f (Django 1.10.dev).

[Jacob Walls]

Bisected this one while having a look at the other squashmigrations tickets.

Fixed in b1cbbe926789bcfc2e118c7d7c7a0f30fab5248a. (#25530)

The regression test included appears to cover this case, so I think closing is appropriate. Let me know if anyone disagrees.