Parent-Child Association

mnuhoglu · **Posted:** Tue Apr 13, 2004 5:14 am

Hello,

I was implementing a parent-child relationship with map. But there was a problem. The index of the map was not stored in the database.

I obeyed the rules explained in the Chapter 9. Parent/Child Relationships of the Hibernate2 Documentation.

The documentation (9.2) says that the relationship between the parent/child should be specified as inverse="true" inside the parent's mapping.

If this is not specified then there will be an INSERT to create the child and an UPDATE to create the link from parent to child. So INSERT will violate the not-null constraint on the parent_id column.

But when I made inverse="false" these problems didn't occur. Moreover the former problem was resolved, namely the index of the map is stored correctly.

My code is:

Code:

        Parent p = (Parent) s.load(Parent.class,pid);
        Child c = new Child();
        p.addChild(c);
        s.save(c);
        s.flush();

Mapping of the parent's map is:

Code:

      <map name="children" table="children" lazy="false" >
         <key column="parentId" />
         <index column="i" type="integer" />
         <one-to-many class="Child" />
      </map>

and the association from child to parent:

Code:

<many-to-one name="parent" class="Parent" not-null="true" column="parentId" />

According to the documentation, the above code should fail. But it works correctly. What is the reason, may anybody help me to understand this?

christian · **Posted:** Thu Apr 15, 2004 7:22 am

You can't used indexed collections or maps with a bidirectional one-to-many association. Either use a Set, a Bag, or make it unidirectional.

mnuhoglu · **Posted:** Fri Apr 16, 2004 3:48 am

Thank you Christian, at first I couldn't give a meaning to what you said. I thought my corrupt solution was working correctly. But then I saw other mistakes in the solution...

I write here the Parent -> Child sample implementation for other people, who encounter the same problem.

The Child class (getter/setters omitted):

Code:

public class Child {
    Integer id;

The Parent class: (Note that Parent sees Child but Child cannot see Parent)

Code:

public class Parent {
    private Map children;
    private Integer id;
    private int lastPosition ;
    public void addChild(Child child){
        lastPosition += 10;
        getChildren().put(new Integer(lastPosition),child);
    }

Parent's mapping:

Code:

   <class name="Parent" table="Parent">
      <id name="id" type="integer" column="id">
         <generator class="native"/>
      </id>
        <property name="lastPosition" column="lastPosition" />
      <map name="children" table="children" lazy="false" cascade="all"  >
         <key column="parentId" />
         <index column="i" type="integer" />
         <one-to-many class="Child" />
      </map>
   </class>

And the test method:

Code:

    public void testMapSavingCascaded() throws HibernateException {
        Serializable pid = new Integer(1);
        Parent p = (Parent) s.load(Parent.class,pid);
        Transaction tx = s.beginTransaction();
        Child c = new Child();
        p.addChild(c);
        tx.commit();
        s.close();
        s = sf.openSession();
        Parent result = (Parent) s.load(Parent.class,pid);
        assertEquals("Parent is not saved",p,result);
    }