one-to-one mapping on the same table

maproskie · **Joined:** Tue Jul 13, 2004 8:54 am **Posts:** 3

I need to map a parent child relationship, to a single table, both parent and child need to use the same primary key and the java code should be generated from the hbm.xml files. I am using hibernate 2.1 and hbm2java code generator

Parent.hbm.xml

Code:

<hibernate-mapping>
    <class name="mapping.Parent" table="MAPPINGTEST">

        <id name="id" type="java.lang.Integer" column="ID">
            <generator class="sequence">
                 <param name="sequence">MAPPINGTEST_ID_SEQ</param>
             </generator>
        </id>
   <property
            name="firstName"
            type="java.lang.String"
            column="FNAME"
            length="25"
        />
        <property
            name="lastName"
            type="java.lang.String"
            column="LNAME"
            length="50"
        />
        <property
            name="age"
            type="java.lang.Integer"
            column="AGE"
            length="11"
        />
    </class>
</hibernate-mapping>

Child.hbm.xml
"This persistent object does not have its own primary key, it should be inherited from the parent"

Code:

<hibernate-mapping>
   <class name="mapping.Child" table="MAPPINGTEST">
        <meta attribute="extends">
            mapping.Parent
        </meta>
        <id type="java.lang.Integer" column="ID">
            <generator class="foreign">
                <param name="table">MAPPINGTEST</param>
                <param name="column">ID</param>
            </generator>            
        </id>
        <one-to-one 
            name="Parent" 
            class="mapping.Parent"
            cascade="none"
            constrained="true"
        />
        <property
            name="firstLineAdd"
            type="java.lang.String"
            column="address1"
            length="30"
        />
        <property
            name="secondLineAddress"
            type="java.lang.String"
            column="address2"
            length="30"
        />
    </class>
</hibernate-mapping>

Is there a way I can get hibernate to use the parent objects <id> field as the child's primary key? What it seems to be doing at this point is creating a new id value instead of inheriting the parents field.
I also can use the <subclass> tag as I don't have a discriminator column available.

Many Thanks
Marc

anthony · **Posted:** Tue Jul 13, 2004 9:32 am

are you sure this is not a many-to-one that you need even if in reality you'll always have only one child?

atorres · **Posted:** Tue Jul 13, 2004 10:41 am

suppouse it is many-to-one that you need even if in reality you'll always have only one child. Is There a way to the O-R abstraction hide it from the class implementation? Do I have to code a Set to represent it !? Is it right? in the conceptual way I think it is. A legacy database misconception should be abstracted in the O-R.
Maybe if he stop using the code generator, and make the class by hand. Is There a way to map 1-1 class relation to 1-n database relationship?
Does it involve complex UserType definitions?
I'm trying to discover how to do that too.

anthony · **Posted:** Tue Jul 13, 2004 10:45 am

many to one doesn't mean there is a set (it is to -one...).
If it is not bidirectionnal, you'll have exactly what you want

shaow db schema and javaBean you expect to have, we'll say if it is possible

gavin · **Posted:** Tue Jul 13, 2004 12:43 pm

I think what you are looking for is <component>

maproskie · **Joined:** Tue Jul 13, 2004 8:54 am **Posts:** 3

OK, I have had a go with the many-to-one child version and have moved forward. Parent.hbm.xml remains the same as above.

child.hbm.xml

Code:

<hibernate-mapping>
    <class name="mapping.Child" table="MAPPINGTEST">
        <meta attribute="extends">
            mapping.Parent
        </meta>
        <id type="java.lang.Integer" column="ID">
            <generator class="foreign">
                <param name="property">Parent</param>
            </generator>            
        </id>
        <many-to-one
            name="Parent" 
            class="mapping.Parent"
            column="ID"
            update="false"
            insert="false"
        />
        <property
            name="firstLineAdd"
            type="java.lang.String"
            column="address1"
            length="30"
        />
        <property
            name="secondLineAddress"
            type="java.lang.String"
            column="address2"
            length="30"
        />
    </class>
</hibernate-mapping>

Hibernate does load the mapping file with no errors, but when I try retrieve the child object I get the following error.

Code:

net.sf.hibernate.WrongClassException: Object with id: 1 was not of the specified subclass: mapping.Child (loaded object was of wrong class)
   at net.sf.hibernate.loader.Loader.instanceAlreadyLoaded(Loader.java:456)
   at net.sf.hibernate.loader.Loader.getRow(Loader.java:423)
   at net.sf.hibernate.loader.Loader.doQuery(Loader.java:209)
   at net.sf.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:133)
   at net.sf.hibernate.loader.Loader.doList(Loader.java:950)
   at net.sf.hibernate.loader.Loader.list(Loader.java:941)
   at net.sf.hibernate.loader.CriteriaLoader.list(CriteriaLoader.java:118)
   at net.sf.hibernate.impl.SessionImpl.find(SessionImpl.java:3571)
   at net.sf.hibernate.impl.CriteriaImpl.list(CriteriaImpl.java:223)
   at com.lbss.framework.persistence.HibernateCriteria.retrieveIterator(HibernateCriteria.java:97)

I will look at using a <component> tag and see how it works.

Many Thanks
Marc

maproskie · **Joined:** Tue Jul 13, 2004 8:54 am **Posts:** 3

The <component> tag does not make use of inheritance as far as I can see, I am using the hbm2java code generator, which adds the parent object to the child as a member variable, I need to make use of Inheritance!

Can anybody help with the error code I am getting in the above post?

Cheers
Marc