one-to-one mapping and lazy loading again

As almost already knows there is no lazy loading for one-to-one for constrained="false" side. This is because NHibernate have to check the other side of relationship if it exists or not.
In some cases we know that the other side always exist from the design of the system.
For such a case attribute like "always-exist" = "true" would cause that NHibernate do not need to check if related object exist. Thus such an attribute enables lazy loading.