Joined-subclass in Hibernate by R4R Team

In Hibernate each subclass can also be mapped to its own table. We called to this is table-per-subclass mapping strategy. An inherited state is retrieved by joining with the table of the superclass. For doing this need to use this <joined-subclass> element. 


For example:


<joined-subclass 

name="ClassName"

table="tablename"

proxy="ProxyInterface"

lazy="true|false"

dynamic-update="true|false" dynamic-insert="true|false" schema="schema" catalog="catalog" extends="SuperclassName" persister="ClassName" subselect="SQL expression" entity-name="EntityName" node="element-name"> <key .... > <property .... /> ..... </joined-subclass>


Here is the description of the all above:


Tag

Description

name

fully qualified class name of the subclass

table

the name of the subclass table

proxy (optional)

specifies a class or interface used for lazy initializing proxies

lazy (optional)

defaults to true.

setting lazy="false" disables the use of lazy fetching


We have to remember a discriminator column is not required for this mapping strategy. Each subclass must, declare a table column holding the object identifier using the <key> element. 

How to write the mapping file it is given below:

<?xml version="1.0"?> 
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="eg"> 
<class name="Cater" table="CATS"> 
<id name="id" column="uid" type="long"> 
<generator class="hilo"/> 
</id> 
<property name="birthdate" type="date"/>
 <property name="color" not-null="true"/> 
<property name="sex" not-null="true"/> 
<property name="weight"/> 
<many-to-one name="mate"/> 
<set name="kittens"> 
<key column="MOTHER"/> 
<one-to-many class="Cater_1"/> 
</set> <joined-subclass name="DomesticCat" table="DOMESTIC_CATS"> 
<key column="CAT"/> 
<property name="name" type="string"/> 
</joined-subclass> 
</class> <class name="eg.Dog"> 
<!-- mapping for Dog could go here --> </class> </hibernate-mapping>
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