Best practice for mapping a one-to-zero-or-one relationship

grainne · **Joined:** Mon Aug 30, 2004 1:37 am **Posts:** 16

Hibernate version: 2.1.6

I understand that it is not possible to lazily load the optional end of a one-to-one (-or zero) association:
http://www.hibernate.org/162.html
especially the last paragraph:

Quote:

f your B->C mapping is mandatory (constrainted=true), Hibernate will use proxy for C resulting in lazy initialization. But if you allow B without C, Hibernate just HAS TO check presence of C at the moment it loads B. But a SELECT to check presence is just inefficient because the same SELECT may not just check presence, but load entire object. So lazy loading goes away.

Given this - is there a best practice for mapping this kind of relationship so that the optional child is not loaded when the parent is loaded? This seems to be a pretty common scenario (a small frequently accessed table with an optional primary key relationship to a much larger rarely accessed table) so I wonder how people are addressing it.

Thanks.
Grainne.

Regis Pires Magalhaes · **Posted:** Tue Oct 18, 2005 4:38 pm

I've been searching a way to do a lazy a "one-to-zero-or-one" relationship in Hibernate and I finally found a way to do it using many-to-one with unique="true". See example below:

Association: Person [1 <--> 0..1] Note

Code:

<class name="Person" table="person">
   <id name="id" column="id" type="int" unsaved-value="-1">
       <generator class="sequence">
           <param name="sequence"father_id_seq</param>
       </generator>
   </id>
   <property name="name" type="string"/>
   <many-to-one name="note" class="Note" column="id" 
                unique="true" insert="false" update="false"
                cascade="all"/>
</class>

<class name="Note" table="note">
   <id name="id" column="id" type="int" unsaved-value="-1" >
       <generator class="foreign">
           <param name="property">owner</param>
       </generator>
   </id> 
   <property name="note" type="string"/>
   <one-to-one name="owner" class="Person" constrained="true"/>
</class>

Observe that column "id" is used twice in Person mapping.