How-to on lazy one-to-one mapping?

jwisard · **Joined:** Tue Oct 19, 2004 11:04 am **Posts:** 22

I have two classes and tables as defined below:

Code:

class A {
    B b;
    String name;
    getB();
    setB();
}

class B {
    A a;
    getA();
    getB();
}

table_a {
    id,
    name
}

table_b {
    id,
    a_id (unique)
}

I want to have a one-to-one relationship betwen A and B via the foreign key.

A<-->B
1 0..1

A can have 0 or 1 B, and B must have 1 A.

So the mappings would be thus:

Code:

<hibernate-mapping>
    <class name="A" table="table_a">
        <id name="id"  type="string">            
            <column name="id" sql-type="CHAR(36)" />
            <generator class="native"/>
        </id>

        <property name="name" type="string">
            <column name="name"/>
        </property> 
       
        <one-to-one name="b" class="B" property-ref="a" cascade="all"/>
    </class>
<hibernate-mapping>

<hibernate-mapping>
    <class name="B" table="table_b">
        <id name="id"  type="string">            
            <column name="id" sql-type="CHAR(36)" />
            <generator class="native"/>
        </id>
 
        <many-to-one name="a" class="A" column="a_id" unique="true" cascade="none" />        
        
    </class>   
</hibernate-mapping>

This works perfectly. My question is, however, 'How do I change this mapping so that when querying instances of A, the B instances are NOT also queried?'. i.e. I want to be able to retrieve instances of A via a Session.get() without having the B members returned. (I want to get the Bs later...)

The retrieval code looks like this:

Code:

Session session  = sessionFactory.openSession();
trans = session.beginTransaction();
A a = (A)session.get(A.class, aId);
trans.commit();
session.close();

if (a.getB() != null) {
    System.out.println("This is bad");
}

I've tried using the lazy="true" attribute of class on both classes for proxying to no avail. I've also tried using various combinations of the outer-join attributes on the associations. Whenever I get A, I always end up getting B as well. In my case, B is a very heavyweight object that only should be retrieved when needed, so this is important (at least to me).

I'm using Hibernate 2.1.2.

jwisard · **Joined:** Tue Oct 19, 2004 11:04 am **Posts:** 22

jwisard wrote:

This works perfectly. My question is, however, 'How do I change this mapping so that when querying instances of A, the B instances are NOT also queried?'. i.e. I want to be able to retrieve instances of A via a Session.get() without having the B members returned. (I want to get the Bs later...)

...or, as an alternative, is there a straight-forward Hibernate-centric way to query A without B, using HQL or the Criteria interface?

vish · **Posted:** Tue Oct 19, 2004 4:05 pm

The one way to do is declate one-to-one mapped class it self as lazy.

<class name="OneToOneRelatio" lazy="true">

Did you take a look at this ?

http://www.hibernate.org/162.html