#24123: LookupError when rolling back migrations
---------------------------------+---------------------
Reporter: MarkusH | Owner: MarkusH
Type: Bug | Status: new
Component: Migrations | Version: master
Severity: Release blocker | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
---------------------------------+---------------------
The `MigrationExecutor.migrate()` method generates the initial project
state from the first migration that is being applied/unapplied. If this
migration has no relations to other apps that are also being (un)applied,
this can lead to `LookupError`s due to missing models:
{{{#!python
Traceback (most recent call last):
File "/home/markus/Coding/django/django/apps/registry.py", line 148, in
get_app_config
return self.app_configs[app_label]
KeyError: 'app_b'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/home/markus/Coding/django/django/db/migrations/state.py", line
181, in __init__
model = self.get_model(lookup_model[0], lookup_model[1])
File "/home/markus/Coding/django/django/apps/registry.py", line 202, in
get_model
return self.get_app_config(app_label).get_model(model_name.lower())
File "/home/markus/Coding/django/django/apps/registry.py", line 150, in
get_app_config
raise LookupError("No installed app with label '%s'." % app_label)
LookupError: No installed app with label 'app_b'.
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "manage.py", line 10, in <module>
execute_from_command_line(sys.argv)
File "/home/markus/Coding/django/django/core/management/__init__.py",
line 338, in execute_from_command_line
utility.execute()
File "/home/markus/Coding/django/django/core/management/__init__.py",
line 330, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/home/markus/Coding/django/django/core/management/base.py", line
390, in run_from_argv
self.execute(*args, **cmd_options)
File "/home/markus/Coding/django/django/core/management/base.py", line
444, in execute
output = self.handle(*args, **options)
File
"/home/markus/Coding/django/django/core/management/commands/migrate.py",
line 213, in handle
executor.migrate(targets, plan, fake=options.get("fake", False))
File "/home/markus/Coding/django/django/db/migrations/executor.py", line
75, in migrate
state = self.unapply_migration(state, migration, fake=fake)
File "/home/markus/Coding/django/django/db/migrations/executor.py", line
130, in unapply_migration
state = migration.unapply(state, schema_editor)
File "/home/markus/Coding/django/django/db/migrations/migration.py",
line 157, in unapply
operation.database_backwards(self.app_label, schema_editor,
from_state, to_state)
File
"/home/markus/Coding/django/django/db/migrations/operations/models.py",
line 62, in database_backwards
model = from_state.apps.get_model(app_label, self.name)
File "/home/markus/Coding/django/django/utils/functional.py", line 60,
in __get__
res = instance.__dict__[self.name] = self.func(instance)
File "/home/markus/Coding/django/django/db/migrations/state.py", line
82, in apps
return StateApps(self.real_apps, self.models)
File "/home/markus/Coding/django/django/db/migrations/state.py", line
191, in __init__
raise ValueError(msg.format(field=operations[0][1],
model=lookup_model))
ValueError: Lookup failed for model referenced by field app_a.A3.b2:
app_b.B2
}}}
Given the attached migration dependencies, the backwards plan for
`('app_a', None)` is
{{{#!python
[
(<Migration app_c.0003_c3>, True),
(<Migration app_a.0004_a4>, True),
(<Migration app_a.0003_a3>, True),
(<Migration app_c.0002_c2>, True),
(<Migration app_b.0003_b3>, True),
(<Migration app_b.0002_b2>, True),
(<Migration app_a.0002_a2>, True),
(<Migration app_a.0001_initial>, True)
]
}}}
The initial state generated from `app_c.0003_c3` is
{{{
app_a.a1:
app_a.a2:
app_c.c1:
app_c.c2:
}}}
As one can see, there are no models of `app_b`, which eventually leads to
an error when unapplying `app_1.0003_a3` which tries to look up model
`app_b.B2`.
--
Ticket URL: <https://code.djangoproject.com/ticket/24123>
Django <https://code.djangoproject.com/>
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