Incorrect migration generated when a JSONField have a local scope defined Decoder

[takuyozora]

Hi,

I have a JSONField on one of my model with a custom JSONDecoder :

class UnitAbstract(models.Model):
    class Meta:
        abstract = True
    # ...
    base_stats = models.JSONField(default=rpg.stats.RPGBaseStats(
        pv={'maximum': 100},
        mana={'maximum': 0}
    ).toJSON(), encoder=RPGJSONEncoder, decoder=rpg.stats.RPGBaseStats.getJSONDecoder())

class Unit(UnitAbstract):
    pass

But the migration file generated by makemigration is incorrect (because of this part : decoder=terrarpg.rpg.mixins.DeconstructibleMixin.getJSONDecoder.<locals>.RPGJSONDecoder) :

class Migration(migrations.Migration):

    dependencies = [
        ('terrarpg', '0035_unitgroup_formation_rank_alter_unit_base_stats_and_more'),
    ]

    operations = [
        migrations.AlterField(
            model_name='unit',
            name='base_stats',
            field=models.JSONField(decoder=terrarpg.rpg.mixins.DeconstructibleMixin.getJSONDecoder.<locals>.RPGJSONDecoder, default=[
#...
], encoder=terrarpg.rpg.parsers.RPGJSONEncoder),
        ),

I assume the <locals> that produce the syntax error is here because my decoder is a dynamically generated class as you can see here :

class DeconstructibleMixin(abc.ABC):
    # ...
    @classmethod
    def getJSONDecoder(cls):
        class RPGJSONDecoder(json.JSONDecoder):
            def decode(self, *args, **kwargs) -> cls:
                data = json.JSONDecoder.decode(self, *args, **kwargs)
                if isinstance(data, list) and len(data) >= 1:
                    return cls.fromJSON(data)
                return data
        return RPGJSONDecoder

If I do not apply the migration everything is working properly (because the field have been created before without the custom decoder).
So maybe either Django should not produce a migration file when only the decoder part change on a JSONField because it's not link to the DB schema, or it should handle properly the fact that the decoder is a local scope defined class.

I hope that the bug issue is filled properly and that my English is compressible enough.

[Yassin Bahri]

I reproduced this issue and prepared a patch with regression tests.

The issue is that migration serialization currently allows local classes to be serialized by TypeSerializer, producing invalid migration code containing <locals>. FunctionTypeSerializer already rejects local functions, so the patch applies the same kind of early failure to local classes.

The regression tests cover both direct local class serialization and the reported JSONField(decoder=...) case.

Tested with:

PYTHONPATH=. python tests/runtests.py migrations.test_writer --verbosity 1 --parallel 1

Branch: ​https://github.com/yassinbahri/django/tree/ticket-37304