'NoneType' object has no attribute 'get_all_fields' parler

If you create your own migration to add fields that must be translatable, you get error:

'NoneType' object has no attribute 'get_all_fields'

1 case. Update parler

In newer versions of parler, this error has been fixed, so it’s enough to upgrade the package.

2 case. Add TranslatableModelMixin in base class of model

Just addMyModel.__bases__ = (models.Model, TranslatableModelMixin) after declaration of MyModel.

For clarity, take an example from the documentation and add the necessary line:

from django.db import migrations
from django.conf import settings
from django.core.exceptions import ObjectDoesNotExist


def forwards_func(apps, schema_editor):
    MyModel = apps.get_model('myapp', 'MyModel')
    MyModel.__bases__ = (models.Model, TranslatableModelMixin)
    MyModelTranslation = apps.get_model('myapp', 'MyModelTranslation')

    for object in MyModel.objects.all():
        MyModelTranslation.objects.create(
            master_id=object.pk,
            language_code=settings.LANGUAGE_CODE,
            name=object.name
        )

def backwards_func(apps, schema_editor):
    MyModel = apps.get_model('myapp', 'MyModel')
    MyModelTranslation = apps.get_model('myapp', 'MyModelTranslation')

    for object in MyModel.objects.all():
        translation = _get_translation(object, MyModelTranslation)
        object.name = translation.name
        object.save()   # Note this only calls Model.save()

def _get_translation(object, MyModelTranslation):
    translations = MyModelTranslation.objects.filter(master_id=object.pk)
    try:
        # Try default translation
        return translations.get(language_code=settings.LANGUAGE_CODE)
    except ObjectDoesNotExist:
        try:
            # Try default language
            return translations.get(language_code=settings.PARLER_DEFAULT_LANGUAGE_CODE)
        except ObjectDoesNotExist:
            # Maybe the object was translated only in a specific language?
            # Hope there is a single translation
            return translations.get()


class Migration(migrations.Migration):

    dependencies = [
        ('yourappname', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(forwards_func, backwards_func),
    ]

The fact is that the migration requires the get_all_fields function, which is available in the TranslatableModelMixin mixin when we inherit our model from TranslatableModel. But when migrating from the base (inherited) classes, only models.Model is used, which is why you need to add TranslatableModelMixin manually.

You can read more about this problem from the comment on the github: https://github.com/django-parler/django-parler/issues/157#issuecomment-386902848.