I basically have the same question Move models between Django (1.8) apps with required ForeignKey references. The response from @halfnibble seems valid but in my case there is a ForeignKey pointing from new_app to old_app. I wil translate my names to names on @halfnibble example for simplicity, Tires will have a ForeignKey to Car.
The migrations runs fine. Few migrations later I rename old_app.Car to old_app.NewCar (I don't know if this affects). But some migrations later when I try to delete new_app.Tires and in next migration old_app.NewCar I get the error:
>python manage.py migrate
....
ValueError: Unhandled pending operations for models:
old_app.newcar (referred to by fields: new_app.Tires.car)
If I run my apps tests they run correctly, I have also inspected generated MySQL queries and seems correct.