Map abstract class using table-per-subclass approach

vishal1shukla2 · **Joined:** Fri Feb 20, 2009 6:25 am **Posts:** 10

Hi all,

I want to map class hierarchy that has abstract class at top level. Can I map it using table-per-subclass approach?

For example,

Code:

abstract class A{

      long id;
      
      //getter & setter for id

}

class B extends A{

     String value;

      //getter & setter for value
}

Please help to map this hierarchy and how should I design tables.

Thanks in advance...

littypreethkr · **Posted:** Fri Feb 20, 2009 7:34 am

Yes you can. I dont see any complications in your example. Your tables would be:

TABLE_A
------------
ID <PK>

TABLE_B
------------
A_ID <PK> <FK refer TABLE_A.ID>
VALUE

Mapping would be:

Code:

<class name="A" table="TABLE_A">
   <id name="id" column="ID" type="long">
      <generator class="native"/>
   </id>
   
   <joined-subclass name="B" table="TABLE_B">
      <key column="A_ID">
      <property name="value" column="VALUE"/>
   </joined-subclass>
</class>

vishal1shukla2 · **Joined:** Fri Feb 20, 2009 6:25 am **Posts:** 10

Hi Letty,

Thanks for the reply.. it certainly workd.. now I had one more addition to it. I hav list of A inside B. and I hav prepared following table for it..

A_B_Map
___________________

id <PK>
a_id <FK_A>
b_id <FK_B>
position

is this table fine to represent that relationship? Can it work without introducing third table?? And I tried to map it but didn't succeed in any... it would be gr8 if sm1 can help me out to map it.

littypreethkr · **Posted:** Sun Feb 22, 2009 8:12 am

What is the type of relation b/w A and B. Is it Many to one or many to many?

If its many to one then you dont need a new a table. Table A can have a FK to table B.

If its many to one then you ll need a separate link table to link A and B.

vishal1shukla2 · **Joined:** Fri Feb 20, 2009 6:25 am **Posts:** 10

Hi litty,

Its many-to-many relationship. and B has many instances of A(List). Can you please help me in creating mapping file for this hierarchy? I am not getting exactly how to map this kind of list.

littypreethkr · **Posted:** Mon Feb 23, 2009 3:05 am

Hey... wait...

At first you said, B is subclass of A. And now you are saying A and B has a many2many relation. This is not possible, remember your table B's primary key is a foreign key referencing table A. So the relation b/w the two is one2one and it can never be many2many.

I think there is some issue with your domain model. Can you give the scenario which you are dealing with.

vishal1shukla2 · **Joined:** Fri Feb 20, 2009 6:25 am **Posts:** 10

Hi Litty,

Actually I am not doing this for some particular domain, but its a kind of generalized pattern which can be used in any specific domains.

For instance, consider that I want to store some table structure with values in the database.

Here consider Cell as Component, Row & Table as Containers.

Try to understand following pattern. I verified this in "in memory" form. But I want to store this table in the database. But I am just blocked at one point, just breaking down the problem, and will post as soon as I identify the same.

Finally, I just want to map this hierarchy in the xml. How do I do that... As its verified "in memory", I believe that it shouldn't be impossible, if it is, then its a bug of Hibernate.

Code:

abstract class Component{

       protected long componentID;
       protected List containers = new ArrayList();

       //Getter and Setters
}

abstract class Container extends Component{
       
       protected List components = new ArrayList();

       //Getter and Setters
}

class Cell extends Component{

      String value;

      //Getter and setters.
}

class Row extends Container{

}

class Table extends Container{

}

class Driver{
      
      public static void main(String[] args){
      
      Cell cell = new Cell();
      cell.setValue("3");

      Row row = new Row();
      row.addComponent(cell);
         
      Table table = new Table();
      table.addComponent(row);
      table.addComponent(row);
      
      Table t2 = new Table();
      t2.addComponent(row);
      t2.addComponent(row);

      }
}

vishal1shukla2 · **Joined:** Fri Feb 20, 2009 6:25 am **Posts:** 10

I am trying following mapping for this.

Code:

<hibernate-mapping>
  <class abstract="true" name="Component" table="component">
    <id column="ComponentID" name="componentID" type="long">
      <generator class="native"/>
    </id>
    <list name="containers" table="container_component_map">
        <key column="ComponentID"/>
        <index column="position"/>
        <many-to-many class="Container" column="ContainerID"/>
    </list>
    <joined-subclass abstract="true" name="Container" table="container">
        <key column="ContainerID"/>
        <list name="components" table="container_component_map">
            <key column="ContainerID"/>
            <index column="position"/>
            <many-to-many class="Component" column="ComponentID"/>
        </list>
        <joined-subclass name="Row" table="row">
            <key column="rowID"/>
            
        </joined-subclass>
        <joined-subclass name="Table" table="table">
            <key column="tableID"/>

        </joined-subclass>
    </joined-subclass>
    <joined-subclass name="Cell" table="cell">
      <key column="cellID"/>
      
      <property column="value" name="value"/>
    </joined-subclass>
  </class>
</hibernate-mapping>

vishal1shukla2 · **Joined:** Fri Feb 20, 2009 6:25 am **Posts:** 10

Hey Litty,

Thanks for reply...

Finally solved the problem... actually the xml file and classes send above works perfectly fine..

only problem was adding "cascade" property for list and the I had put ArrayList(not List) inside Setter of the components & containers.

Thanks again....

littypreethkr · **Posted:** Mon Feb 23, 2009 6:58 am

May be I didn't really understood ur domain model... But isn't it invalid that the same component can be present in more than one container? Also is it really a valid condition that the same row can occur more than once in a table?

Anyways you have different tables for TABLE, ROW, CELL etc. So rather than making MTM relation b/w container and component isnt it better to have MTO relation from table2row and row2cell?