models.py

class EntityItself(models.Model):
    """Все что мы хотим от нее - уникальный PrimaryKey"""
    
    def current_state(self):
        return self.entitystate_set.latest()

    def __getattr__(self, attr):
        return getattr(self.current_state(), attr)

class EntityState(models.Model):
    entity = models.ForeignKey(EntityItself)
    version = models.DateTimeField()

    title = models.CharField()
    # ... everything else