Mapping an interface hierarchy with subclasses

elophant · **Posted:** Thu Dec 11, 2003 7:15 pm

Hello... i have a problem on how to map the following simple example scenario.

The Hibernate Reference tells me to use the <subclass>-Tag to map implementig classes of interfaces. But I also need the <subclass>-Tag to express the polymorphic structure, that is i. e. "Question" is extended by "NumericQuestion"

Is the following mapping correct?

Code:

<hibernate mapping>
    <class name="Question" table="QUESTIONS" discriminator-value="Question">
        <id name="Id" column="ID">
            <generator class="native" />
        </id>
        <property name="questionText" column="QUESTION_TEXT" />
        <discriminator column="CLASS_NAME" />

        <!-- Now the implementing class -->
        <subclass name="QuestionDTO" discriminator-value="QuestionDTO" />

        <!-- Now the subinterface -->
        <subclass name="NumericQuestion" discriminator-value="NumericQuestion">
            <property name="minValue" column="MIN_VALUE" />
            <property name="maxValue" column="MAX_VALUE" />

            <!-- Now the implementig class of this interface -->
            <subclass name="NumericQuestionDTO" discriminator-value="NumericQuestionDTO" />

        </subclass>
    </class>
</hibernate-mapping>

I am concerned about the lines of code in which the DTOs (stands for "Data Transfer Object") are mapped. It's just one line, where I say what's the name of the implementing class and what discriminator-value I give to it. Is that enough?

I know that later in the running application, its the DTO-Objects, that will be persisted to the database, because you can't create an object of an interface, right? But I'd like the type of the DTOs to be the corresponding interface-type, because one should only access theses DTOs through their interfaces.

How does Hibernate understand, that in one case the <subclass>-Tag is used to declare an implementing class and in another case to declare a real subclass (subinterface in this case)?

For all help best thanks in advance!

Philipp Hinnah

emmanuel · **Posted:** Fri Dec 12, 2003 7:03 am

If you save a real object ie a DTO, Hibernate will know that DTO is a DTO getClass().

If you request 'from Interface', hibernate will only find real DTO impl in discriminator column, so DTO will be instanciated

elophant · **Posted:** Fri Dec 12, 2003 7:42 am

Hello epbernard!

I did not really understand, what you mean with "request 'from interface'".

Philipp Hinnah

emmanuel · **Posted:** Fri Dec 12, 2003 8:46 am

in a HQL query, you can do 'from my.Interface' and it will return object id DB implementing this interface since you mapped it.

elophant · **Posted:** Fri Dec 12, 2003 9:01 am

Aha...!

So when I specifiy the interface in the from-clause the query will return an Object ID of a DTO-Object, that is implementing this interface?

emmanuel · **Posted:** Fri Dec 12, 2003 9:09 am

Something like that.
It will return the discriminator value and all needed properties, instanciate the appropriate implementation then filling it in

elophant · **Posted:** Fri Dec 12, 2003 12:02 pm

I found another approach how to solve this problem, that I have two parallel subclass-hierarchies (one with the interfaces and one with the DTOs):

I just map the DTOs and use the interfaces as proxies! Then it'll work fine I guess :-)

P. Hinnah