meaning of discriminator-value attribute of class tag

y25zhao · **Joined:** Thu May 08, 2008 2:16 pm **Posts:** 3

Hi,

I'm a beginner of Hibernate. Could anyone provide me an example of how discriminator-value attribute work? I didn't get it from online tutorial. Thanks for help.

cbredesen · **Posted:** Tue May 13, 2008 4:06 pm

Try the manual:

http://www.hibernate.org/hib_docs/v3/re ... criminator

"The <discriminator> element is required for polymorphic persistence using the table-per-class-hierarchy mapping strategy and declares a discriminator column of the table. The discriminator column contains marker values that tell the persistence layer what subclass to instantiate for a particular row. A restricted set of types may be used: string, character, integer, byte, short, boolean, yes_no, true_false."

You won't need to use this until you map subclasses to a single table. Discriminator value lets Hibernate know which type a given row is.

sepgs2004 · **Joined:** Tue May 13, 2008 10:01 am **Posts:** 8

Following is a description that is been taken (extract) from Java Persistence book. Credit goes to the authors. I just worded a different similar example.

This is a case that I know where it can be used. Suppose we have two (it can be as many as possible) classes (that has some common/general properties) in Java and one table to represent this in database. I mean we would like to persist both the objects into a single table.

Example scenario:

Code:

public abstract class Attachment {
  - String name;
  - ETC
}

public ImageAttachement extends Attachment {
  - Integer imageSizeWidth;
  - Integer imageSizeHeight;
  - String imageType;
}

public TextAttachment extends Attachment {
  - Integer size;
}

We have only one table (to represent this class hierarchy) called
attachment (attachment_id number, type varchar(1), name varchar(25), image_size_width number, image_size_height number, image_type varchar(3), size number)

Hibernate mapping:

Code:

<class 
        name="Attachment"
        table="attachment">
        <id
            name="id"
            column="attachment_id"
            type="long">
            <generator class="native"/>
        </id>
        <discriminator 
            column="attachment_type"
            type="string"/>
        <property 
            name="sentBy"
            column="sent_by"
            type="string"/>
            ...
        <subclass 
            name="ImageAttachment"
            discriminator-value="IMG">
            <property name="imageSizeWidth" column="image_size_width"/>
            <property name="imageSizeHeight" column="image_size_height"/>
            <property name="imageType" column="image_type"/>
        </subclass>

        <subclass
            name="TextAttachment"
            discriminator-value="TXT">
        ...

    </class>

Since we are storing two objects into a single table, for persistence and retrieval purposes, since Hibernate handles this automatically and internally, we have to have a special column in the table to distinguish between these two persistent classes: the discriminator.

Please note that there is NO such property in the persistent classes. This discriminator is used internally by Hibernate. Hibernate will set correct values ('IMG" or "TXT") for this discriminator column automatically.

Hope this helps.

sepgs2004 · **Joined:** Tue May 13, 2008 10:01 am **Posts:** 8

Mentioning of this discriminator starts from the hibernate mapping file and there is a corresponding column in the table. One can see why we need this. This is all is necessary since Hibernate does retrieval, persistence, internally.

So mostly this is to help Hibernate in correctly mapping the two classes into one table.

sepgs2004 · **Joined:** Tue May 13, 2008 10:01 am **Posts:** 8

Mentioning of this discriminator starts from the hibernate mapping file and there is a corresponding column in the table. One can see why we need this. This is all is necessary since Hibernate does retrieval, persistence, internally.

So mostly this is to help Hibernate in correctly mapping the two classes into one table.

There is no mention of it in the Java classes.