Classes embedded inside other classes are not serialized correctly

[Dan Loewenherz]

Simple example:

class Store(models.Model):
    @deconstructible
    class IgnoreFormatString(object):
        def __init__(self, s):
            self.s = s

        def __str__(self):
            return self.s

        def __mod__(self, k):
            return self

    name = models.CharField(max_length=64, null=True)
    rating = models.IntegerField(error_messages={'invalid': IgnoreFormatString("You've entered an invalid value.")}, null=True)

The migration is created appropriately, but running it will fail with something like the following:

Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "my_project/lib/python2.7/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
    utility.execute()
  File "my_project/lib/python2.7/site-packages/django/core/management/__init__.py", line 377, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "my_project/lib/python2.7/site-packages/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "my_project/lib/python2.7/site-packages/django/core/management/base.py", line 338, in execute
    output = self.handle(*args, **options)
  File "my_project/lib/python2.7/site-packages/django/core/management/commands/migrate.py", line 63, in handle
    executor = MigrationExecutor(connection, self.migration_progress_callback)
  File "my_project/lib/python2.7/site-packages/django/db/migrations/executor.py", line 17, in __init__
    self.loader = MigrationLoader(self.connection)
  File "my_project/lib/python2.7/site-packages/django/db/migrations/loader.py", line 48, in __init__
    self.build_graph()
  File "my_project/lib/python2.7/site-packages/django/db/migrations/loader.py", line 173, in build_graph
    self.load_disk()
  File "my_project/lib/python2.7/site-packages/django/db/migrations/loader.py", line 103, in load_disk
    migration_module = import_module("%s.%s" % (module_name, migration_name))
  File "/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "my_project/app/migrations/0001_initial.py", line 8, in <module>
    class Migration(migrations.Migration):
  File "my_project/app/migrations/0001_initial.py", line 19, in Migration
    ('rating', models.IntegerField(null=True, error_messages={b'invalid': app.models.IgnoreFormatString(b"You've entered an invalid value.")})),
AttributeError: 'module' object has no attribute 'IgnoreFormatString

Let me know if you need anything else to help diagnose the issue or have any questions. Cheers!

[Andrew Godwin]

Unfortunately, this is unserializable, as there's no way to get the reference to the surrounding class from the inner class. I'll update the docs to mention this, but the only fix for you is to move InvalidFormatString to the outer scope.

[Andrew Godwin <andrew@…>]

In 45768e6b72607b4bc913358c560830454f3336a7:

Update docs to be more clear about nested classes (refs #23418)

[Andrew Godwin <andrew@…>]

In e5cdfb151039cdd22a17480f0f1fb9ccbad5eb56:

[1.7.x] Update docs to be more clear about nested classes (refs #23418)

[Dan Loewenherz]

@Andrew Any way we can prevent the migration itself from being created as well? Although the docs mention it now, the migration is still created and users won't encounter the error until after running it. Thoughts?

[Dan Loewenherz]

Reopening since I'm not convinced a doc-only change is sufficient. I'm happy to contribute a patch if needed.

[Markus Holtermann]

I opened a pull-request: ​https://github.com/django/django/pull/3194

[Dan Loewenherz]

Awesome, thanks!

[Markus Holtermann <info@…>]

In d28b5f13b332bda4317949b98e33f528d30ec006:

Fixed #23418 -- Fail when migration deconstruct produces invalid import

[Tim Graham <timograham@…>]

In b0def3bcac1a2e1117050e00fa1b187a897f0c2e:

[1.7.x] Fixed #23418 -- Fail when migration deconstruct produces invalid import

Backport of d28b5f13b3 from master

[Tim Graham <timograham@…>]

In 16548cfc7c9d86ae7b0d13425ec7608f0571130c:

Fixed broken test from da160d440f; refs #23418.

[Tim Graham <timograham@…>]

In 1f1a32928873a4e7cfe2f8f15631aa63df335490:

[1.7.x] Fixed broken test from da160d440f; refs #23418.

Backport of 16548cfc7c from master

[Tim Graham <timograham@…>]

In fbeb7979ee43cb386265ff7aec757e35b4cf61f8:

Avoided using deprecated version of importlib; refs #23418.

[Tim Graham <timograham@…>]

In 4500784b24856b23924e7d36b405f2df1499ea83:

[1.7.x] Avoided using deprecated version of importlib; refs #23418.

Backport of fbeb7979ee from master