Issue with joined-subclass

brunopalaoro · **Joined:** Fri Feb 17, 2006 9:10 am **Posts:** 4

Hibernate version:
3.1
Mapping documents:

Code:

    <class name="User" table="users" schema="public">
        
        <id name="cod">
            <column name="userid" not-null="true"/>
            <generator class="sequence">
               <param name="sequence">usuarios_usuarioid_seq</param>
            </generator>
        </id>
        
        <property name="name" type="string">
            <column name="name" />
        </property>
        
        <property name="passwd" type="string">
            <column name="passwd"  />
        </property>
               
    </class>

   <joined-subclass name="Teacher" table="teacher" extends="User">
   
      <key column="userid" />
            
      <bag name="colDisciplinas" table="disciplinasprofessores" lazy="true">
         <key column="userid" />
         <many-to-many class="Discipline" column="disciplineid"/>
      </bag>
   
   </joined-subclass>

Hi there!
First, sorry if my english is bad.
Now, the problem.
I'm having an issue with a joined-subclass. I have an user table and a teacher table. A teacher is also a user, so Teacher extends User, and I'm using the table-per-subclass strategy. Also there's many-to-many mapping in the Teacher class.
It's working smootly when loading is involved, so I assume there's no critical error in the mapping.
The thing is when I try to insert a teacher.
In the system, a teacher register itself, so I insert it in the user table. The system don't know yet if this user will be a teacher.
After that, the admin of the system can change the user role, marking him as a teacher, so I insert a line in the teacher table. But when I do that, Hibernate also insert a line in the user table, assuming it's a new user, but that's not the case, since I already have an user with that id. As a result of this, I get 2 lines in the user table with the same data.
Is there a way to make Hibernate think 'look, I already have an user with that id, so I will just insert it in the teachers table'??

In any case, thanks in advance :).

TOMBATLECRUISEr · **Joined:** Tue Nov 29, 2005 12:31 pm **Posts:** 75

Hi,

I know I don't answer to your question, but you could delete the old user record from database before inserting the new teacher record in database.

the-gtm · **Joined:** Wed Feb 08, 2006 3:59 pm **Posts:** 75

TOMBATLECRUISEr wrote:

Hi,

I know I don't answer to your question, but you could delete the old user record from database before inserting the new teacher record in database.

Given the problem, I think this is the correct thing to do.

When you mark an user as teacher, what you're implicitly doing is changing an object's class which is bad IMO. A better way to do it would be to add a roles table. Alternatively you could use a single-table hierarchy : users and teachers in the same table with a discriminator column. Then you could update one's discriminator to make it a teacher. Still this is "class changing" : you've got to be extra carefull, with caches and detached objects for instance.

brunopalaoro · **Joined:** Fri Feb 17, 2006 9:10 am **Posts:** 4

Hi.

Deleting the old user worked, but this do not make me happy, because I believe is a very ugly way to solve the problem, since it will also delete all the records related to that user.

Quote:

When you mark an user as teacher, what you're implicitly doing is changing an object's class which is bad IMO. A better way to do it would be to add a roles table.

Actually I have a roles table (used for Tomcat security and stuff), but the thing is that a teacher has more attributes then an user, and I can't think of another solution for this, avoiding the class changing, as you say. And is impossible in this case to create a teacher right from the start, as the system doesn't know who really is a teacher until the admin says so.

I think I'll need to do the single-table hierarchy. It's not an aproach I like (the user table will be enormous), but it's the better solution right now IMO.

Anyone can give me another ideia about how to deal with this, or the single-table hierarchy is my solution?

stewart.cambridge · **Joined:** Sat Nov 19, 2005 2:46 pm **Posts:** 69

Thank you for raising this question. I have a similar heirarchy where an Organization has many Person objects. Some Person objects should be Advsior objects, which have more attributes. Finally a User is an Advisor who also has a logon/password.
So my heirarchy is Person<-Advisor<-User.
I am on the edge of hitting the same problem, and I don't know if it will work but my intention was to:

Code:

Advisor fred = getFred(); // this advisor already exists
Advisor newFred = new User(fred); // this copy constructor will carry over all data such as contact phone number, email, etc
session.delete(fred); // I really need to do this?
session.update(newFred); // will this work? or I need to session.save()?

I did not come across this problem yet, because I hit a different one first:

Code:

organization.getAdvisors(); // is returning all Persons - I want only Advisors
organization.getUsers(); // is returning all Persons - I want only Users

I cannot find the right mapping to make this work... :-(

brunopalaoro · **Joined:** Fri Feb 17, 2006 9:10 am **Posts:** 4

In my tests, I've loaded an user, copied his information to a teacher object, and then tried this:
1- delete the user and insert a new teacher: worked, but all the data with foreign keys refering to the user table was also deleted (and it's the right thing to do in this case, since I really deleted an user).
2 - delete the user and update the teacher: no way, Hibernate throws a NonUniqueObjectException (associate two different instances of the same java class with a particular identifier, in the scope of a single Session (this is in the javadoc)).
3 - just update the teacher: not worked also: throws a StaleObjectStateException with the message Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect).

So, I'm still in the some point where all started, lol.

And about you're loading problem, did you really mapped as a joined-sublass? Once i've had the same problem as you, and the problem was that in the java code I extended , but mapped as two separeted classes.
Look your mappings and see if helps.

TOMBATLECRUISEr · **Joined:** Tue Nov 29, 2005 12:31 pm **Posts:** 75

Well, you've tryed to solve the inheritance problem using table-per-subclass solution. How about using table-per-hierarchy solution ? Maybe this is the right stategy for your problem.

If this new approach doesn't fit to your requests, maybe you should transform your is-a relation to a has-a relation ( teacher is a user will be user has a teacher <or not>).