Sortable form collection quandary

[lsmoker]

I'd like to use drag-and-drop sorting in a form collection. Unfortunately, the primary key field that is updated by the sorting operation is also referenced in another (dependent or child) model in a ManyToManyField (ForeignKey would also be a problem). So when sorting changes the primary key, the relationship to the other model is broken.

For example...

class ParentCompany(models.Model):
    name = models.TextField()

class Manufacturer(models.Model):
    parentcompany = models.ForeignKey(ParentCompany, on_delete=models.CASCADE)

class Car(models.Model):
    manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE)

Using drag-and-drop sorting in a form collection on the Manufacturer model could leave orphans in the Car model since primary keys that Car refers to would be changed.

Is there a way to use a sort field other than the primary key like the https://github.com/jrief/django-admin-sortable2 package does?