Bases: nova.objects.base.NovaPersistentObject, nova.objects.base.NovaObject, nova.objects.base.NovaObjectDictCompat
Bases: nova.objects.base.ObjectListBase, nova.objects.base.NovaObject
It only makes sense to call this method when the BlockDeviceMappingList contains BlockDeviceMappings from exactly one instance rather than BlockDeviceMappings from multiple instances.
For example, you should not call this method from a BlockDeviceMappingList created by get_by_instance_uuids(), but you may call this method from a BlockDeviceMappingList created by get_by_instance_uuid().