#35652: Unapplying a data migration that removes data fails with relations
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Reporter: Timothy Schilling | Type: Bug
Status: new | Component: Migrations
Version: 5.0 | Severity: Normal
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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I believe there's a bug in the plan logic for the `MigrationExecutor`
class. When a related model has been removed in an earlier migration, a
later data migration that removes data from a model will attempt to run a
query to collect/delete data from the related model. Unfortunately, that
related model's table has been removed from the database.
Given the following app and model structure:
{{{
# App A
class A(models.Model):
name = models.CharField()
# App B
class B(models.Model):
a = models.ForeignKey(A, on_delete=models.CASCADE)
}}}
1. Create migrations
2. Remove model `B`, create migration
3. Apply all migrations forwards
4. `python manage.py makemigrations a --empty --name create_data`
{{{
def create_data(apps, schema_editor):
A = apps.get_model('a', 'A')
A.objects.create(name='test')
def remove_data(apps, schema_editor):
A = apps.get_model('a', 'A')
A.objects.filter(name='test').delete()
# In migration class
operations = [migrations.RunPython(create_data, remove_data)]
}}}
5. Attempt to reverse back to A 0001 (`python manage.py migrate a 0001`)
It should break on the reverse of A 0002_create_data, because it will
attempt to run a query to delete related `B` instances, but that table has
been removed.
It appears that the migration app state isn't removing the `B` model.
I was able to track this down to at least the
`MigrationExecutor.migration_plan` method returning a full plan that puts
the all app A migrations before the second app B migration.
I found that the RemoveModel operation mutates the state properly, but
that `MigrationExecturor._migrate_all_backwards` is using a different
state from `states[migration]`. That state is from the `full_plan` that
gets passed in which comes from `MigrationExectutor.migration_plan`.
Interestingly enough, if we change this part of the code:
https://github.com/django/django/blob/main/django/db/migrations/executor.py#L195-L214
To
{{{
for migration, _ in full_plan:
if not migrations_to_run:
# We remove every migration that we applied from this set so
# that we can bail out once the last migration has been applied
# and don't always run until the very end of the migration
# process.
break
if migration not in migrations_to_run and migration in
applied_migrations:
# Only mutate the state if the migration is actually applied
# to make sure the resulting state doesn't include changes
# from unrelated migrations.
migration.mutate_state(state, preserve=False)
for migration, _ in full_plan:
if not migrations_to_run:
# We remove every migration that we applied from this set so
# that we can bail out once the last migration has been applied
# and don't always run until the very end of the migration
# process.
break
if migration in migrations_to_run:
if "apps" not in state.__dict__:
state.apps # Render all -- performance critical
# The state before this migration
states[migration] = state
# The old state keeps as-is, we continue with the new state
state = migration.mutate_state(state, preserve=True)
migrations_to_run.remove(migration)
}}}
It unapply successfully because all the `applied_migrations` are mutating
the state before the `migrations_to_run` stores any state.
Note: I haven't reproduced this on a fresh project.
--
Ticket URL: <https://code.djangoproject.com/ticket/35652>
Django <https://code.djangoproject.com/>
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