many-to-many self-referencing association

rmonson · **Joined:** Thu Aug 19, 2004 3:15 pm **Posts:** 4

I want to do this:

Person <---------> Person

If Person A is a friend of Person B, than B is a friend of A

A.getFriends().add(B);

List listA = a.getFriends();

List listB = b.getFriends();

ListA will contain Person B
List B will contain Pereson A.

Is this possible in Hibernate - I've read the forums and the docs but this is not addressed. Can't figure out how to do it.

anthony · **Posted:** Thu Aug 19, 2004 3:38 pm

just read about many to many...

rmonson · **Joined:** Thu Aug 19, 2004 3:15 pm **Posts:** 4

Well, I've read it again and still can't figure it out. The problem is the relationship is not just self-referencing, many-to-many its also bidirectional. Please take a look at my first post if this is not clear.

Anyhelp - other than pointing to the documentation again - would be greatly appreciated.

anthony · **Posted:** Thu Aug 19, 2004 4:35 pm

if you want help

Quote:

- other than pointing to the documentation again -

,
read the red card and propose, at least, a document mapping

rmonson · **Joined:** Thu Aug 19, 2004 3:15 pm **Posts:** 4

OK. Hopefully this will help:

I have only one entity in my data base, person. Here is the mapping for Person.

Code:

  <class name="com.orkut.Person" table="PERSON">
    <meta attribute="class-description">
      Represents a single Person in the Orkut Database.
      @author RMH
    </meta>

    <id name="id" type="int" column="PERSON_ID">
      <meta attribute="scope-set">protected</meta>
      <generator class="native"/>
    </id>

    <property name="firstName" type="string" not-null="true"/>

    <property name="lastName" type="string" not-null="true"/>
        
        <set name="friends" table="FRIENDS_LINK" >
   <key column="person_1_id"/>
            <many-to-many class="com.orkut.Person" column="person_2_id"/>
         </set>

  </class>

The class generated from this looks like this:

Code:

package com.orkut;

import java.io.Serializable;
import java.util.Set;
import org.apache.commons.lang.builder.ToStringBuilder;


/** 
 *       Represents a single Person in the Orkut Database.
 *       @author RMH
 *     
*/
public class Person implements Serializable {

    /** identifier field */
    private Integer id;

...

    /** persistent field */
    private Set friends;

  ...
    public Set getFriends() {
        return this.friends;
    }

    public void setFriends(Set friends) {
        this.friends = friends;
    }

    ...

}

I run one script to populate my database. Its very simple and it works

Code:

            tx = session.beginTransaction();

            Person rrose = new Person("Richard","Rose",new java.util.HashSet());
            session.save(rrose);

            Person dWater = new Person("David","Water",new java.util.HashSet());
            session.save(dWater);

            Person dSteel = new Person("David","Steel",new java.util.HashSet());
            session.save(dSteel);

            Person dWood = new Person("Don","Wood",new java.util.HashSet());
            session.save(dWood);

            rrose.getFriends().add(dWater);
            rrose.getFriends().add(dSteel);
            rrose.getFriends().add(dWood);
            dWater.getFriends().add(dSteel);
            dSteel.getFriends().add(dWood);

            // We're done; make our changes permanent
            tx.commit();

I than run a script to query the database. Its also simple:

Code:

            for (Iterator iter = session.find("from com.orkut.Person").iterator() ; iter.hasNext() ; ) {
                Person person = (Person)iter.next();
                outName(person.getFirstName()+" "+person.getLastName());
                for(Iterator friends = person.getFriends().iterator();friends.hasNext(); ){
                  Person friend = (Person)friends.next();
         outName("   "+friend.getFirstName()+" "+friend.getLastName());
                }
            }

The output from this query is as follows:

Code:

qtest:
     [java] Richard Rose
     [java]    David Steel
     [java]    David Water
     [java]    Don Wood
     [java] David Water
     [java]    David Steel
     [java] David Steel
     [java]    Don Wood
     [java] Don Wood

However, if the relationship was bidirectional than it should have looked like this (bold items were missing):

Code:

qtest:
     [java] Richard Rose
     [java]    David Steel
     [java]    David Water
     [java]    Don Wood
     [java] David Water
     [java]    David Steel
     [java]    Richard Rose
     [java] David Steel
     [java]    Don Wood
     [java]    Richard Rose
     [java] Don Wood
     [java]    David Steel
     [java]    Richard Rose