How to properly map nested subclasses using hibernate

Question

How do I nest subclasses with Hibernate?

For example, I have the following class hierarchy:

PageElement
- PageParagraph
- PageImage
- PageVideo
- PageAudio
- PageQuestion
  - PageFillInTheBlankQuestion
  - PageOpenEndedQuestion
  - PageMultipleChoiceQuestion
  - ...

This is the ideal object structure. How can I successfully map this using Hibernate? I would say that the PageQuestion abstract class is VERY important. It contains a lot of reusable properties and logic. I also need to reference PageQuestion specifically while not referencing PageElement <- this would be bad. I'd also like to query against all PageQuestion objects.

score 1 · Answer 1 · answered Jul 03 '13 at 17:47

In Hibernate 4.0 (maybe earlier), you can use nested elements if you are using the table-per-hierarchy model.

Example class structure:

Word (has property "text", string)
- Subject (adds property "plural", boolean)
  - Pronoun (adds property "masculine", boolean)
  - ProperNoun (adds property "person", boolean)
- Verb (adds property "past_tense", boolean)

Example Hibernate mapping XML:

<class name="test.Word" table="words">
    <id name="id"><generator class="native"/></id>
    <discriminator column="word_type" type="string"/>
    <property name="text"/>
    <subclass name="test.Subject">
        <property name="plural"/>
        <subclass name="test.ProperNoun">
            <property name="person"/>
        </subclass>
        <subclass name="test.Pronoun">
            <property name="masculine"/>
        </subclass>
    </subclass>
    <subclass name="test.Verb">
        <property name="past_tense"/>
    </subclass>
</class>

Then, after running the following code (I've left out the class code, it's very basic and honestly only the object types themselves are significant for this example):

    HibernateUtil.beginTransaction();
    HibernateUtil.getCurrentSession().persist(new Word("blue"));
    HibernateUtil.getCurrentSession().persist(new Subject("chairs", true));
    HibernateUtil.getCurrentSession().persist(new ProperNoun("James", true));
    HibernateUtil.getCurrentSession().persist(new Pronoun("he", false, true));
    HibernateUtil.getCurrentSession().persist(new Verb("sat", true));
    HibernateUtil.commitTransaction();

The database ends up containing this information:

id, word_type, text, plural, person, masculine, past_tense,
1, test.Word, blue, NULL, NULL, NULL, NULL
2, test.Subject, chairs, 1, NULL, NULL, NULL
3, test.ProperNoun, James, 0, 1, NULL, NULL
4, test.Pronoun, he, 0, NULL, 1, NULL
5, test.Verb, sat, NULL, NULL, NULL, 1

And querying the list of objects back results in the proper types:

    HibernateUtil.beginTransaction();
    List<Word> words = HibernateUtil.getCurrentSession().createCriteria(Word.class).list();
    for (Word w:words)
        System.out.println(w);
    HibernateUtil.commitTransaction();

Prints:

test.Word@caf6c1
test.Subject@10e35d5
test.ProperNoun@18e8541
test.Pronoun@1ce85c4
test.Verb@17aece8

See section 10.1.1 of http://docs.jboss.org/hibernate/core/4.0/manual/en-US/html/inheritance.html (it is essentially the same as the old docs though), although unfortunately it does not clearly indicate that nested subclasses are allowed -- but it is an easy thing to experiment with.

I agree that the Hibernate documentation is very poorly organized. Everything is there but I find that something about the way it is organized makes it very difficult for me to find complete information; mostly due to over use of cross-references, and a use-case oriented layout that doesn't always cover all the bases. However, the information is there.

score 0 · Answer 2 · answered Dec 11 '11 at 14:08

0

Thats exactly the issue between Object and Relational paradigm.

Hibernate is the tool for this. And there you have the reference do already for this.

http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html_single/#d0e6906

answered Dec 11 '11 at 14:08

manocha_ak

904
7
19

I figured out how to do it with a table-per-hierarchy pattern. It is odd that hibernate file looks nothing like the examples shown in that document. The document is not exhausted enough. – egervari Dec 12 '11 at 14:21

How to properly map nested subclasses using hibernate

2 Answers2