Delete all the objects in the database that are not in objects_to_keep.
        - objects_to_keep: A map where the keys are classes, and the values are a
         set of the objects of that class we should keep.

def remove_objects_not_in(self, objects_to_keep, verbosity):
        """
        Delete all the objects in the database that are not in objects_to_keep.
        - objects_to_keep: A map where the keys are classes, and the values are a
         set of the objects of that class we should keep.
        """
        for class_ in objects_to_keep.keys():
            current = class_.objects.all()
            current_ids = set([x.pk for x in current])
            keep_ids = set([x.pk for x in objects_to_keep[class_]])

            remove_these_ones = current_ids.difference(keep_ids)
            if remove_these_ones:
                for obj in current:
                    if obj.pk in remove_these_ones:
                        obj.delete()
                        if verbosity >= 2:
                            print("Deleted object: %s" % six.u(obj))

            if verbosity > 0 and remove_these_ones:
                num_deleted = len(remove_these_ones)
                if num_deleted > 1:
                    type_deleted = six.u(class_._meta.verbose_name_plural)
                else:
                    type_deleted = six.u(class_._meta.verbose_name)

                print("Deleted %s %s" % (str(num_deleted), type_deleted))