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wetrull

Post subject: How to get only super class in table-per-subclass strategy?

Posted: Tue Apr 18, 2006 3:29 pm

Newbie

Joined: Tue Apr 18, 2006 2:50 pm
Posts: 10

I'm trying to get instances of the super class in a "table per subclass"
inheritance mapping strategy but have not been successful. My core
questions are:

1) Is it possible to get instances of only the super class?

2) Is it possible to tell Hibernate to not add "left outer join"s to the
super class select statements (see below)?

I've looked at the test/joinedsubclass JUnit test case and feel I have a
similar situation. Where I differ is not wanting to fetch the subclass
data unless explictly requested because the subclass contains BLOB
data. Additionally, there will always be a row in the subclass table
for each row in the super class table.

I've read Hibernate In Action, done extensive Googling and searched
the Hibernate web site, but have not found an answer. Maybe I need a
paradigm shift in my thinking/implementation.

Thanks.

-Eric Trull

Hibernate version:
3.1.3

Mapping documents:

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.xxx.yyy">

  <!-- Data store document description -->
  <class name="DocumentDescription" table="DB_FILE" lazy="true">
  
    <!--Primary key-->
    <id name="primaryKey" column="DB_FILE_ID" type="long">
      <generator class="sequence">
         <param name="sequence">DB_FILE_SEQ</param>
      </generator>
    </id>

    <natural-id>
      <!-- File name -->
      <property name="name" column="NAME" type="string" length="256"
                not-null="true" update="false" />

      <!-- File content type -->
      <property name="type" column="FILE_TYPE" type="string"
                     length="50" not-null="true" update="false" />
                
      <!-- File date -->
      <property name="created" column="CREATED_DATE"
                type="com.xxx.hibernate.type.ForcedJavaUtilDateType"
                not-null="true" update="false" />
    </natural-id>

  </class>

</hibernate-mapping>

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.xxx.yyy">

  <!-- Data store document description plus compressed content -->
  <joined-subclass name="Document" extends="DocumentDescription" 
                   table="DB_FILE_DATA" lazy="true">
    <!-- Foreign key column -->
    <key column="DB_FILE_ID" />
      
    <!-- Document compressed content -->
    <property name="compressedContent" column="COMPRESSED_DATA"
              type="binary" not-null="true" lazy="true" />
  </joined-subclass>

</hibernate-mapping>

Code between sessionFactory.openSession() and session.close():
DocumentDescription is a simple POJO. Document extends
DocumentDescription and adds methods to compress/de-compress
the BLOB data. I'm using a DocumentDescriptionHome generated
by Hibernate Tools. I've removed CUD methods (leaving the R
(find) methods) from DocumentDescriptionHome and will rely on
DocumentHome for CUD.

I've tried modifying DocumentDescriptionHome to use a HQL
statement (from DocumentDescription as description where
description.primaryKey=:primaryKey) instead of the Criteria API
without success.

Code:

public class DocumentDescriptionHome {

    private Session getCurrentSession() {
        // using thread local pattern
        return HibernateUtility.getSessionFactory().getCurrentSession();
    }

    public DocumentDescription findById(Long id) {
        log.debug("getting DocumentDescription instance with id: " + id);
        try {
            DocumentDescription instance = (DocumentDescription) getCurrentSession()
                    .get(DocumentDescription.class, id);
            if (instance==null) {
                log.debug("get successful, no instance found");
            } else {
                log.debug("get successful, instance found");
            }
            return instance;
        } catch (RuntimeException re) {
            log.error("get failed", re);
            throw re;
        }
    }

    ...

    // for testing only
    public static void main(String[] args) {
        DocumentDescriptionHome home = new DocumentDescriptionHome();
        DocumentDescription description;
        
        HibernateUtility.getSessionFactory().getCurrentSession()
                .beginTransaction();
        
        description = home.findById(new Long(2668L));
        System.out.println(description);
        if (description instanceof Document) {
            try {
                System.out.println(
                    new String(((Document) description).getContent()));
            } catch (IOException exception) {
                // TODO: Handle IOException
                exception.printStackTrace();
            }
        }
        
        HibernateUtility.getSessionFactory().getCurrentSession()
                .getTransaction().rollback();
        HibernateUtility.getSessionFactory().getCurrentSession().close();
    }
}

Full stack trace of any exception that occurs:

Name and version of the database you are using:
Oracle 9

The generated SQL (show_sql=true):

Debug level Hibernate log excerpt:

http://hibernate.sourceforge.net/hibern ... on-3.0.dtd

http://hibernate.sourceforge.net/hibern ... ng-3.0.dtd

Last edited by wetrull on Wed Apr 19, 2006 5:29 pm, edited 1 time in total.

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r1ch

Post subject:

Posted: Wed Apr 19, 2006 7:45 am

Regular

Joined: Wed Aug 25, 2004 6:23 am
Posts: 91

Yes - just enable explicit polymorphism for your classes:

http://www.hibernate.org/hib_docs/v3/re ... tion-class

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wetrull

Post subject:

Posted: Wed Apr 19, 2006 5:27 pm

Newbie

Joined: Tue Apr 18, 2006 2:50 pm
Posts: 10

Thanks for the reply r1ch. It does not appear that setting polymorphism to explicit in the base class mapping works. I've found other postings asking the same sort of question with regards to joined-subclass (table-per-subclass strategy) without any resolution.

I've tried both

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.xxx.yyy">

  <!-- Data store document description -->
  <class name="DocumentDescription" table="DB_FILE" polymorphism="explicit">
  
    <!--Primary key-->
    <id name="primaryKey" column="DB_FILE_ID" type="long">
      <generator class="sequence">
         <param name="sequence">DB_FILE_SEQ</param>
      </generator>
    </id>

    <natural-id>
      <!-- File name -->
      <property name="name" column="NAME" type="string" length="256"
                not-null="true" update="false" />

      <!-- File content type -->
      <property name="type" column="FILE_TYPE" type="string" length="50"
                not-null="true" update="false" />
                
      <!-- File date -->
      <property name="created" column="CREATED_DATE"
                type="com.xxx.hibernate.type.ForcedJavaUtilDateType"
                not-null="true" update="false" />
    </natural-id>

  </class>

</hibernate-mapping>

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.xxx.yyy">

  <!-- Data store document description plus compressed content -->
  <joined-subclass name="Document" extends="DocumentDescription" 
                   table="DB_FILE_DATA">
    <!-- Foreign key column -->
    <key column="DB_FILE_ID" />
      
    <!-- Document compressed content -->
    <property name="compressedContent" column="COMPRESSED_DATA"
              type="binary" not-null="true" />
  </joined-subclass>

</hibernate-mapping>

-- and --

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.xxx.yyy">

  <!-- Data store document description -->
  <class name="DocumentDescription" table="DB_FILE" polymorphism="explicit">
  
    <!--Primary key-->
    <id name="primaryKey" column="DB_FILE_ID" type="long">
      <generator class="sequence">
         <param name="sequence">DB_FILE_SEQ</param>
      </generator>
    </id>

    <natural-id>
      <!-- File name -->
      <property name="name" column="NAME" type="string" length="256"
                not-null="true" update="false" />

      <!-- File content type -->
      <property name="type" column="FILE_TYPE" type="string" length="50"
                not-null="true" update="false" />
                
      <!-- File date -->
      <property name="created" column="CREATED_DATE"
                type="com.xxx.hibernate.type.ForcedJavaUtilDateType"
                not-null="true" update="false" />
    </natural-id>

    <!-- Data store document description plus compressed content -->
    <joined-subclass name="Document" table="DB_FILE_DATA">
      <!-- Foreign key column -->
      <key column="DB_FILE_ID" />
      
      <!-- Document compressed content -->
      <property name="compressedContent" column="COMPRESSED_DATA"
                type="binary" not-null="true" />
    </joined-subclass>

  </class>

</hibernate-mapping>

I've also looked at the Lightweight pattern (http://www.hibernate.org/41.html), but that does not use the joined-subclass specifier.

Can anybody confirm that polymorphism="explicit" works with joined-subclass?

Thanks.

-Eric Trull

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r1ch

Post subject:

Posted: Wed Apr 19, 2006 5:41 pm

Regular

Joined: Wed Aug 25, 2004 6:23 am
Posts: 91

Hi Eric - I'm fairly sure that our app used to count on this functionality (we use joined-subclass too).

In what way isn't it working - are you getting an error or anything?

Note that Hibernate's first level cache means that you can't have two instances of the same object in the same session, so you if you load a Document with a given id first then loading a DocumentDescription with the same id will give you the original Document back and vice-versa. In a clean session you should be able to load the object as either.

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wetrull

Post subject:

Posted: Wed Apr 19, 2006 6:05 pm

Newbie

Joined: Tue Apr 18, 2006 2:50 pm
Posts: 10

R1ch,

No, I'm not getting any exceptions or errors. What I mean by it not working is that when I execute DocumentDescriptionHome.main (see original post) I get back an instance of Document (with the fetched BLOB data) instead of an instance of DocumentDescription. I'm confirming this in my debugger as well as the fact that the BLOB contents are printed out. I know that a Document has not already been loaded because the only code being executed is in main.

If you have the time and ability I'd appreciate it if you could look at your app's mapping files and compare them to the ones I've posted.

BTW, here are the un-resolved posts I was refering to: http://forum.hibernate.org/viewtopic.php?t=926622), and http://forum.hibernate.org/viewtopic.php?t=927708.

Thanks!

-Eric Trull

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lmiranda

Post subject:

Posted: Wed Apr 19, 2006 7:37 pm

Beginner

Joined: Mon Nov 29, 2004 2:26 pm
Posts: 28

The way it works is that you don't map Document as a subclass, just a separate class that happens to point to the same table. So instead of using extends="DocumentDescription", just map it using polymorphism="explicit" and the same table:

Code:

  <!-- Data store document description -->
  <class name="DocumentDescription" table="DB_FILE">
 
    <!--Primary key-->
    <id name="primaryKey" column="DB_FILE_ID" type="long">
      <generator class="sequence">
         <param name="sequence">DB_FILE_SEQ</param>
      </generator>
    </id>
    
    <natural-id>
      <!-- File name -->
      <property name="name" column="NAME" type="string" length="256"
                not-null="true" update="false" />

      <!-- File content type -->
      <property name="type" column="FILE_TYPE" type="string" length="50"
                not-null="true" update="false" />
               
      <!-- File date -->
      <property name="created" column="CREATED_DATE"
                type="com.xxx.hibernate.type.ForcedJavaUtilDateType"
                not-null="true" update="false" />
    </natural-id>

    </class>

and

Code:

<!-- Data store document description plus compressed content -->
    <class name="Document" table="DB_FILE" polymorphism="explicit">
      <!-- Foreign key column -->
      <key column="DB_FILE_ID" />

 
    <!--Primary key-->
    <id name="primaryKey" column="DB_FILE_ID" type="long">
      <generator class="sequence">
         <param name="sequence">DB_FILE_SEQ</param>
      </generator>
    </id>
    
    <natural-id>
      <!-- File name -->
      <property name="name" column="NAME" type="string" length="256"
                not-null="true" update="false" />

      <!-- File content type -->
      <property name="type" column="FILE_TYPE" type="string" length="50"
                not-null="true" update="false" />
               
      <!-- File date -->
      <property name="created" column="CREATED_DATE"
                type="com.xxx.hibernate.type.ForcedJavaUtilDateType"
                not-null="true" update="false" />
    </natural-id>

      <!-- Document compressed content -->
      <property name="compressedContent" column="COMPRESSED_DATA"
                type="binary" not-null="true" />
    </joined-subclass>

  </class> 

By the way, Hibernate 3 supports lazy loading of individual properties, which would be more transparent to use.

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wetrull

Post subject:

Posted: Wed Apr 19, 2006 8:27 pm

Newbie

Joined: Tue Apr 18, 2006 2:50 pm
Posts: 10

Thanks for the reply Imiranda. What you have shown is what is basically described in the Lightweight pattern (http://www.hibernate.org/41.html). If push comes to shove, I will switch to that approach.

However, I've avoided that approch because, IMO, it begins to smell of duplicate maintence in the mapping file. My object extends the super class, so "in my little world" the subclass mapping file should "extend" the super class mapping file (by using joined-subclass). What would really make me happy (I think) is to have joined-subclass have the polymorphic attribute.

Supposing that polymorphic was available for joined-sublcass, what would be the significant disadvantage(s) of this approach over using the Lightweight pattern (two class mapping files)?

Thanks.

-Eric Trull

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lmiranda

Post subject:

Posted: Thu Apr 20, 2006 6:15 am

Beginner

Joined: Mon Nov 29, 2004 2:26 pm
Posts: 28

wetrull wrote:

However, I've avoided that approch because, IMO, it begins to smell of duplicate maintence in the mapping file. My object extends the super class, so "in my little world" the subclass mapping file should "extend" the super class mapping file (by using joined-subclass). What would really make me happy (I think) is to have joined-subclass have the polymorphic attribute.

I agree, I don't like it either. If I were you I would:
- factor the BLOB out into a separate entity, and use a lazily-loaded association, or
- look into lazy loading of properties (Hibernate 3)

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wetrull

Post subject:

Posted: Thu Apr 20, 2006 11:42 am

Newbie

Joined: Tue Apr 18, 2006 2:50 pm
Posts: 10

lmiranda wrote:

If I were you I would:
- factor the BLOB out into a separate entity, and use a lazily-loaded association, or
- look into lazy loading of properties (Hibernate 3)

More great solutions Imiranda, thanks. Your first solution is similar to what I'm trying to do by using joined-subclass.

In both solutions, however, these objects are vended by a web service (SOAP over HTTP) using a session-per-request pattern. Thus, by the time the objects are serialized into XML for the response all the lazy loaded associations and properties get loaded.

I think by using the joined-subclass I'm trying to allow programatic loading of a lazily initialized bit of data. When the code asks for DocumentDescription, the BLOB shouldn't be loaded. When the code asks for a Document I explicitly want the BLOB loaded.

Anybody see a reason that polymorphic="explicit" should not be allowed on joined-subclass?

Thanks.

-Eric Trull

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lmiranda

Post subject:

Posted: Thu Apr 20, 2006 12:54 pm

Beginner

Joined: Mon Nov 29, 2004 2:26 pm
Posts: 28

wetrull wrote:

In both solutions, however, these objects are vended by a web service (SOAP over HTTP) using a session-per-request pattern. Thus, by the time the objects are serialized into XML for the response all the lazy loaded associations and properties get loaded.

That's not strictly true--serialization alone of a Hibernate proxy does not initialise the proxy. Hence the dreaded LazyInitializationException.

Only if you access any of the entitie's fields will it be initialised. See http://www.hibernate.org/hib_docs/v3/re ... ng-proxies

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wetrull

Post subject:

Posted: Thu Apr 20, 2006 1:40 pm

Newbie

Joined: Tue Apr 18, 2006 2:50 pm
Posts: 10

lmiranda wrote:

Yes, you are correct, *Java* serialization will not initialize the proxy. However, the SOAP framework's serialization (over used term here) of the proxy into XML will and does initialize the proxy.

Remember that in a session-per-request pattern a session is opened for each request, the Hibernate unit-of-work is performed, a response created, and then the session is closed right before the response is sent. In this case, the SOAP framework "serializes" the Hibernate object(s) into an XML document which is placed in the SOAP message. In doing so, the SOAP framework calls all the get methods on the Hibernate proxy object resulting in a full initialization of the proxy. Specifying lazy in the mapping file does not help because the SOAP framework ends up loading everything.

To add in another tanget, there is certainly room for debate about the web service granularity and whether the POJOs should be passed back or some other object. This is an ongoing debate but for the moment we have found a happy place.

Thanks.

-Eric Trull

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wetrull

Post subject: org.hibernate.MappingException: UniqueKey _UniqueKey alread

Posted: Thu Apr 27, 2006 1:07 pm

Newbie

Joined: Tue Apr 18, 2006 2:50 pm
Posts: 10

Based on suggestions here, I switched to the Lightweight pattern. My mapping files now look like:

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.xxx.yyy">

  <!-- Data store document description -->
  <class name="DocumentDescription" table="DB_FILE" polymorphism="explicit"
         proxy="DocumentDescription">
  
    <!--Primary key-->
    <id name="primaryKey" column="DB_FILE_ID" type="long">
      <generator class="sequence">
         <param name="sequence">DB_FILE_SEQ</param>
      </generator>
    </id>

    <natural-id>
      <!-- File name -->
      <property name="name" column="NAME" type="string" length="256"
                not-null="true" update="false" />

      <!-- File content type -->
      <property name="type" column="FILE_TYPE" type="string" length="50"
                not-null="true" update="false" />
                
      <!-- File date -->
      <property name="created" column="CREATED_DATE"
                type="com.xxx.hibernate.type.ForcedJavaUtilDateType"
                not-null="true" update="false" />
    </natural-id>

  </class>

</hibernate-mapping>

and

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.xxx.yyy">

  <!-- Data store document, including content -->
  <class name="Document" table="DB_FILE" polymorphism="explicit"
         proxy="Document">
  
    <!--Primary key-->
    <id name="primaryKey" column="DB_FILE_ID" type="long">
      <generator class="sequence">
         <param name="sequence">DB_FILE_SEQ</param>
      </generator>
    </id>

    <natural-id>
      <!-- File name -->
      <property name="name" column="NAME" type="string" length="256"
                not-null="true" update="false" />

      <!-- File content type -->
      <property name="type" column="FILE_TYPE" type="string" length="50"
                not-null="true" update="false" />
                
      <!-- File date -->
      <property name="created" column="CREATED_DATE"
                type="com.xxx.hibernate.type.ForcedJavaUtilDateType"
                not-null="true" update="false" />
    </natural-id>

    <join table="DB_FILE_DATA">
      <!-- Foreign key column -->
      <key column="DB_FILE_ID" />
      
      <!-- Document compressed content -->
      <property name="compressedContent" column="COMPRESSED_DATA" type="binary"
                not-null="true" />
    </join>
    
  </class>

</hibernate-mapping>

However, I now get a org.hibernate.MappingException, claiming that "UniqueKey _UniqueKey already exists!". If I remove the <natural-id> declaration from DocumentDescription (but leave the properties) the exception goes away. This seems like a bug to me. Does anybody disagree?

Thanks.

-Eric Trull

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