django-modeltranslation: Django 1.10 default manager not MultilingualManager for model inheriting abstract class with a manager defined on objects (leading to AttributeError: 'QuerySet' object has no attribute 'rewrite')

Whilst upgrading my application to Django 1.10 I came across the following issue where the default manager for a Model Translation registered model didn’t get dynamically set to modeltranslation.manager.MultilingualManager.

This resulted in an exception being thrown in update_translation_fields.py L32

AttributeError: 'QuerySet' object has no attribute 'rewrite'

A similar issue was resolved in #123 but it seems something in the internal changes in Django 1.10 has resurfaced it in the particular scenario I have (not one covered by #381).

I haven’t yet identified what the changes are that cause it but I’ve narrowed the scenario down to one in which a model inherits from an abstract model that explicitly sets a manager via the objects attribute. In this case the resulting _default_manager is that set on objects rather than modeltranslation.manager.MultilingualManager.

The simplest example I can boil it down to is this:

# models.py

class FooAbstract(models.Model):
    class Meta:
        abstract = True

class Foo(FooAbstract):
    name = models.CharField(blank=True, max_length=1)

class Bar(models.Model):
    objects = models.Manager()
    name = models.CharField(blank=True, max_length=1)

class BazAbstract(models.Model):
    objects = models.Manager()
    class Meta:
        abstract = True

class Baz(BazAbstract):
    name = models.CharField(blank=True, max_length=1)


# translations.py

@register(Foo)
class FooTranslationOptions(TranslationOptions):
    fields = ('name',)

@register(Bar)
class BarTranslationOptions(TranslationOptions):
    fields = ('name',)

@register(Baz)
class BazTranslationOptions(TranslationOptions):
    fields = ('name',)


# shell

from modeltranslation.translator import translator
models = translator.get_registered_models(abstract=False)
for model in models:
    print(model.__name__, type(model._default_manager))

# Django 1.9 output:

Foo <class 'modeltranslation.manager.MultilingualManager'>
Bar <class 'modeltranslation.manager.MultilingualManager'>
Baz <class 'modeltranslation.manager.MultilingualManager'>

# Django 1.10 output:

Foo <class 'modeltranslation.manager.MultilingualManager'>
Bar <class 'modeltranslation.manager.MultilingualManager'>
Baz <class 'django.db.models.manager.Manager'>

I don’t have a solution yet but thought I’d submit it as an issue now in case anyone has any ideas?