NHibernate: how to enable lazy loading on one-to-one mapping

There is way thought. It's described here in details :

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernateTest" namespace="NHibernateTest">
  <class name="Person" >
    <id name="PersonID" type="Int32">
      <generator class="identity" />
    </id>
    <property name="LastName" type="String" length="50" />
    <property name="FirstName" type="String" length="50" />
    <many-to-one name="Photo" class="PersonPhoto" />
  </class>

  <class name="PersonPhoto">
    <id name="PersonID" type="Int32">
      <generator class="foreign">
        <param name="property">Owner</param>
      </generator>
    </id>
    <property name="Photo" type="BinaryBlob" />
    <one-to-one name="Owner" class="Person" constrained="true" />
  </class>
</hibernate-mapping> 

As far as I know, there isn't a non-hacky way to lazy load a one-to-one. I hope I'm wrong, but last time I checked it was the case.

Lazy loading of one-to-one isn't supported unless the association is mandatory. See here for the reasoning.

It boils down to the fact that in order to decide if the other side of the relationship exists (N)Hibernate has to go to the database. Since you've already taken the database hit, you might as well load the full object.

While there are cases where hitting the DB just to see if the related object exists without actually loading the object makes sense (if the related object is very "heavy"), it isn't currently supported in NHibernate.