Consider for a moment a Blogging application where a blog owner has a custom list of Tags they can use. So I have a Person entity that has a list of Tag entities. Each person in the system has their own unique list of Tags, so a Tag also refers back to the Person that owns it.

Obviously, this is simple enough to do with hibernate (in fact, I already have that working). But now I need to do this with the different elements in separate projects, where one project doesn't know about the other one. And I'm running into troubles.

Here is an overview of the two projects I'm working with for this example (using Hibernate 3.3):

First project: Common

• Contains common classes used in many other projects
• Does not know about second project (Application project below) -- application.jar is not included
• Contains an abstract common.entity.Identity class
  
  Code:
  public abstract class Identity implements Serializable {
      private Long id;
      private String first;
      private String middle;
      private String last;
  }
  
• Contains a concrete common.entity.Tag class which has an Identity property
  
  Code:
  public class Tag implements Serializable
  {
      private Long id;
      private String name;
      private Identity identity;
  }
  
• Contains a mapping file Tag.hbm.xml
  
  Code:
  <hibernate-mapping package="common.entity" default-access="field">
      <class name="Tag" table="tag">
          <cache usage="read-write"/>
          <id name="id" column="id">
              <generator class="identity"/>
          </id>
  
          <property name="name" length="10"/>
          <many-to-one name="identity" not-null="true" cascade="none" />
       </class>
  </hibernate-mapping>
  

Second project: Application

• An application that includes the common.jar file (Common project above)
• Contains a concrete application.entity.Person class that extends common.entity.Identity
  
  Code:
  public class Person extends Identity
  {
      private String username;
      private String password;
      private List<Tag> tags = new ArrayList<Tag>();
  }
  
• Contains a mapping file Person.hbm.xml
  
  Code:
  <hibernate-mapping package="application.entity" default-access="field">
      <class name="Person" table="person">
          <cache usage="read-write"/>
          <id name="id" column="id">
              <generator class="identity"/>
          </id>
  
          <property name="first" length="25"/>
          <property name="middle" length="25"/>
          <property name="last" length="50"/>
          <property name="username" length="50"/>
          <property name="password" length="50"/>
  
          <list name="tags" table="personTag">   
              <key column="person" />
              <list-index column="idx" />
              <many-to-many class="common.entity.Tag" column="tag" />
          </list>
      </class>
  </hibernate-mapping>

Problem code
When I attempt to populate some test data (using a Spring InitializingBean), I get an exception. Here is the code:


Here is the exception:


My Questions

• Is it possible to have the Tag.hbm.xml mapping file refer to the abstract class Identity?
• Do I need to move the Tag.hbm.xml file into the Application project and change it to refer to a Person instead of an Identity?
• Is there a proper way to accomplish what I'm trying to do using Inheritance mapping? The examples in the manual don't seem to match what I'm doing. Person is the only class that extends Identity in the application (other applications will extend Identity differently), so I only need one Person table, not multiple tables like can be used with inheritance mapping.
• Can anyone give me examples of how to do this correctly? I'm a bit confused on the proper solution to this.

If you need any further information, please let me know. Note that I've simplified the code and mappings above to make this post easier to understand. Also, please don't ask me to move my Person object from Application to Common or Tag to Application, as that will not work for my use case. Unless no solution exists, I need to retain this project structure.

Thank you in advance!
Tauren

OMG... I need to head to bed.

It looks like my problem wasn't related to abstract classes or inheritance or anything like that. I just wasn't saving my objects in the right order.

I realized that person hadn't been saved yet when the tag was being saved, which explains the "not-null property references a null or transient value". So I moved personDao.save(person) to before the for() loop and it all works.



Thanks,
Tauren

Optimizations:

1. Specify 'cascade' attribute for the Person.tags association and remove unnecessary calls to tagDao.save(tag);
2. Introduce tag.setIdentity(this); to the person.addTag(tag) and remove that explicit call from the dao;

Thanks, these are both great optimizations and I've applied both to my code.

Tauren