Discriminator in a joined-subclass

cherouvim · **Joined:** Fri May 12, 2006 9:58 am **Posts:** 32

Hibernate version:
3.1.3

Mapping documents:

Code:

    <class name="jc.Event" table="events">
        <id name="id" column="id"><generator class="native"/></id>
        <property name="title"/>
        
        <joined-subclass name="jc.Meeting" table="meetings">
            <key column="event_id"/>

            <discriminator column="type"/>
            
            <property name="meetingDetails"/>
            
            <subclass name="jc.FooMeeting" discriminator-value="FOO" />
            <subclass name="jc.BarMeeting" discriminator-value="BAR" />
        </joined-subclass>
        
    </class>

Full stack trace of any exception that occurs:
The content of element type "joined-subclass" must match "(meta*,subselect?, synchronize*,comment?,tuplizer*, key,(property|many-to-one|one-to-one| component|dynamic-component|properties| any|map|set|list|bag|idbag|array|primitive-array)*, joined-subclass*,loader?,sql-insert?,sql-update?, sql-delete?,resultset*,(query|sql-query)*)".

The mapping above is not valid. I cannot use a discriminator in a joined-subclass element.
How can I map this hierarchy of classes? I have a Meeting class which extends the Event class. The Meeting class has some subclasses which do not have any extra fields so I want to discriminate their run-time type using column "type". How is this possible?

Should I use more joined-subclass elements without any property elements inside? I feel that it's not worth to do a database join just for a subclass which does not add anything to the base class.

thanks

tenwit · **Joined:** Thu Dec 23, 2004 9:08 pm **Posts:** 2008

If you have a discriminator, then use <subclass>, not <joined-subclass>. If you have to join to another table once you've figure out which subclass you're using, then use the <join> tag inside the <subclass> tag.

DanielSerodio · **Posted:** Tue Apr 08, 2008 11:30 am

tenwit wrote:

If you have a discriminator, then use <subclass>, not <joined-subclass>. If you have to join to another table once you've figure out which subclass you're using, then use the <join> tag inside the <subclass> tag.

Can you please elaborate on this? How do you join to another table once you've figure out which subclass you're using?

I came to this forum looking for the following answer: if I don't need to specify a discriminator value for each (joined) subclass, then how does Hibernate know which class to instantiate when reconstructing an object from the DB?

tenwit · **Joined:** Thu Dec 23, 2004 9:08 pm **Posts:** 2008

DanielSerodio wrote:

Can you please elaborate on this? How do you join to another table once you've figure out which subclass you're using?

Inside the subclass' mapping, use the <join> mapping.

DanielSerodio wrote:

if I don't need to specify a discriminator value for each (joined) subclass, then how does Hibernate know which class to instantiate when reconstructing an object from the DB?

If a row in the superclass table has a corresponding row in the joined-subclass table "A", then it's an instance of class A. If instead it has a row in joined-subclass table "B", then it's an instance of class B.

Using the example in section 9.1.2 of the refdocs (section number may have changed, I have an old version of the refdocs):

Code:

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="AMOUNT"/>
    ...
    <joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </joined-subclass>
    <joined-subclass name="CashPayment" table="CASH_PAYMENT">
        <key column="PAYMENT_ID"/>
        ...
    </joined-subclass>
    <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        <key column="PAYMENT_ID"/>
        ...
    </joined-subclass>
</class>

Given an instance of the Payment superclass, with ID 1, hibernate can check the CREDIT_PAYMENT, CASH_PAYMENT and CHEQUE_PAYMENT tables. It finds value 1 in the PAYMENT_ID column of the CHEQUE_PAYMENT table, then it knows that Payment instance 1 is a ChequePayment.