Re: Inheritance: One Table

[Thomas Dudziak]

Tino Sch�llhorn wrote:
Hi,
I have I inheritance-problem. I want to map two classes to one table. 
They are defined as followed:

class A {
    int id;
    protected String ojbConcreteClass;
    // some attributes   
}

class B extends A {
    // some additional attributes
}
I created class-descriptors for each of the table:
<class-descriptor class="A" table="table">
    // field-descriptors for A inluding ojbConcreteClass
</class>
and
<class-descriptor class="B" table="table" extends="A">
    // additional field-descriptors for B
</class>
So: When I try to store an instance of Type A everything is fine. But 
when I try to store an instance of Type B OJB complains:

Caused by: 
org.apache.ojb.broker.metadata.ClassNotPersistenceCapableException: OJB 
needs at least one primary key attribute for class 
objectsRealClass=class kos.wnetz.om.RelationHistoryEvent, 
objectTopLevelClass=class kos.wnetz.om.RelationHistoryEvent
    at 
org.apache.ojb.broker.Identity.checkForPrimaryKeys(Identity.java:245)
    at org.apache.ojb.broker.Identity.init(Identity.java:121)

So I also tried to include the field-descriptors of the superclass in 
the class-descriptor for class B. But then I get an even worse error:

Caused by: java.sql.SQLException: Parameter index out of range (8 > 7).
I don't know where this exception comes from, but in order to describe a 
super-subtype relationship you should use something like:

<class-descriptor class="A" table="table">
    <extent-class class-ref="B"/>
    // field-descriptors for A inluding ojbConcreteClass
</class>
and
<class-descriptor class="B" table="table">
    // field-descriptors for B including ojbConcreteClass
</class>
The extent-class is inverse to the extends/implements in Java (i.e. 
specified at the supertype), and also note that the class-descriptor for 
B contains all attributes of A (there is no automatic 'inheritance' of 
the descriptors in the repository.xml file) including the 
ojbConcreteClass attribute.

Tom
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