Makemigrations tries to migrate non-model classes

[Christian Peters]

If your model extends non-model classes, the migration fails with:

raise ValueError("Cannot serialize: %r\nThere are some values Django cannot serialize into migration files.\nFor more, see ​https://docs.djangoproject.com/en/dev/topics/migrations/#migration-serializin

The reason is that before adding the base classes there is no check if the base class is indeed a django model:

​https://github.com/django/django/blob/master/django/db/migrations/autodetector.py#L495-505

Solution: Check if the base class is an instance of django.db.model before adding it.

Context:

I have Models with some mixins that just add method and functionality that is shared among multiple models, e.g. an InvoiceMixin, that is shared by multiple models to enrich them with convenience methods.

[Christian Peters]

It turned out, that I just need to inherit the mixin from object to make it work in django 1.7