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Parent/Child with just one table (composite keys, too!)

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saimonsez

Post subject: Parent/Child with just one table (composite keys, too!)

Posted: Fri Oct 21, 2005 9:21 am

Newbie

Joined: Fri Oct 21, 2005 4:31 am
Posts: 8
Location: Bern / Switzerland

Hi all,

I'm new to Hibernate and have the following problem: Basically, all I'd like to map is a Parent/Child relationship on a legacy app. The table "Tree" looks like

---------------------------------------------------------
| id1 | id2 | name | parent_id1 | parent_id2 |
---------------------------------------------------------

where id1 and id2 is a composite key and parent_id1 and parent_id2 are foreign keys to the same table (a parent has multiple childs).

The goal is to get a Java Object tree with the root element containing all its direct children in a collection "children" (and so forth). Then I'd like to export this tree to a XML document.

So far I have a TreeId class for the composite key, a Tree class with getters and setters for treeId, name and children (collection) and this Tree.hbm.xml:

Code:

...

  <class name="Tree">
  
    <composite-id name="id" class="TreeId">
      <key-property name="id1"/>
      <key-property name="id2"/>
    </composite-id>
    
    <set name="children" table="Tree" inverse="true" cascade="all-delete-orphan">
      <key>
        <column name="parentId1" not-null="true"/>
        <column name="parentId2" not-null="true"/>
      </key>
      <one-to-many class="Tree"/>
    </set>
    
    <property name="name"/>
    
  </class>
...

This produces empty foreign keys on inserts. Is there any chance to make this work? If so, how would performance be?

BTW, if this table was a nested Set, could hibernate efficiently handle that? (just curious)

Thanks for all answers!

Regards,
Simon

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ashneyde

Post subject:

Posted: Fri Oct 21, 2005 10:28 am

Beginner

Joined: Tue Nov 25, 2003 3:33 pm
Posts: 35

this is what worked for me:

Code:

package domain;

import java.util.HashSet;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratorType;
import javax.persistence.Id;
import javax.persistence.IdClass;
import javax.persistence.JoinColumn;
import javax.persistence.JoinColumns;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.Transient;
import org.hibernate.annotations.Cascade;
import edu.columbia.law.tas.CompositeKey;

@Entity
@Table(name="csample")
@IdClass(value=CompositeKey.class)
public class CompositeIdEntity {
    
    private CompositeKey key = new CompositeKey ();
    private String name;
    private Set<CompositeIdEntity> children;
    private CompositeIdEntity parent;
    
    public CompositeIdEntity () {}
    public CompositeIdEntity (CompositeKey key) {
        this.key = key;
    }

    @OneToMany(cascade=CascadeType.ALL,fetch=FetchType.EAGER,mappedBy="parent")
    @Cascade(value=org.hibernate.annotations.CascadeType.DELETE_ORPHAN)
    public Set<CompositeIdEntity> getChildren () { return children; }
    public void setChildren (Set<CompositeIdEntity> children) { this.children = children; }

    @Id(generate=GeneratorType.NONE)
    @Column(insertable=false,updatable=false)
    public CompositeKey getKey () { return key; }
    public void setKey (CompositeKey key) { this.key = key; }

    public String getName () { return name; }
    public void setName (String name) { this.name = name; }

    @ManyToOne
    @JoinColumns({
        @JoinColumn (name="pid1"),
        @JoinColumn (name="pid2")
    })
    public CompositeIdEntity getParent () { return parent; }
    public void setParent (CompositeIdEntity parent) { this.parent = parent; }
    
    public void addChild (CompositeIdEntity child) {
        if (this.getChildren() == null) {
            this.setChildren(new HashSet<CompositeIdEntity> ());
        }
        child.setParent(this);
        this.getChildren().add(child);
    }
    
    @Transient
    public Long getId1 () { return getKey().getId1(); }
    public void setId1 (Long id1) { key.setId1 (id1); }
    
    @Transient
    public void setId2 (Long id2) { key.setId2 (id2); }
    public Long getId2 () { return key.getId2 (); }
    
    
}

then test will look like this:

Code:

CompositeIdEntity p = new CompositeIdEntity (new CompositeKey(new Long (0), new Long (0)));
p.setName("Parent");

CompositeIdEntity c0 = new CompositeIdEntity (new CompositeKey(new Long (0), new Long (1)));
c0.setName("Child 0");
                        
CompositeIdEntity c1 = new CompositeIdEntity (new CompositeKey(new Long (0), new Long (2)));
c1.setName("Child 1");
                        
p.addChild(c0);
p.addChild(c1);

session.saveOrUpdate(p);

and SQL DDL:

Code:

DROP TABLE IF EXISTS `hibernate-goofing`.`csample`;
CREATE TABLE `csample` (
  `id1` int(10) unsigned NOT NULL default '0',
  `id2` int(10) unsigned NOT NULL default '0',
  `name` varchar(45) default NULL,
  `pid1` int(10) unsigned default '0',
  `pid2` int(10) unsigned default '0',
  PRIMARY KEY  (`id1`,`id2`)
);

worked charmingly well :-)

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jd001982

Post subject: Re: Parent/Child with just one table (composite keys, too!)

Posted: Fri Oct 21, 2005 9:22 pm

Beginner

Joined: Tue Oct 18, 2005 3:57 pm
Posts: 48
Location: Los Angeles, CA

saimonsez wrote:

This produces empty foreign keys on inserts. Is there any chance to make this work? If so, how would performance be?

Just allow NULL for parent key columns:

Code:

    <set ... >
      <key>
        <column ... not-null="false"/>
        <column ... not-null="false"/>
      </key>
    </set>

saimonsez wrote:

BTW, if this table was a nested Set, could hibernate efficiently handle that? (just curious)

Don't worry about Hibernate's efficiency because:
1) you can always set lazy="true", and
2) if there's any performance issues, chances are they won't be in Hibernate. :)

jd

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saimonsez

Post subject:

Posted: Mon Oct 24, 2005 5:03 am

Newbie

Joined: Fri Oct 21, 2005 4:31 am
Posts: 8
Location: Bern / Switzerland

Thanks for the replies! Unfortunately, I have to wait until next weekend to test it (loads of work ...).

I've never worked with these ejb tags, but I guess that the Hibernate Ant Tools are needed to generate XML configs?

Regards,
Simon

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