Persons & Friends -- newbie mapping question

-don · **Joined:** Mon May 09, 2005 5:26 pm **Posts:** 21

Hello,

can anyone explain to me how one might implement the classic "Person/Friend" problem using Hibernate?

in relational land a "Person" table has an ID and some info and a "Friend" table is a self-join of the "Person" table -- that is, the "Friend" table has two colums holding the ID's of two persons. in object land, a class Person has a field "friends" of type Set, into which are placed objects of type Person.

I'm *far* from a Hibernate wiz, but a person on our development team who is using Hibernate claims it can't be done -- I find this difficult to believe.

Bonus question: anyone know how to do the above using the new java/hibernate annotations?

thanks much,

-don.

vgiguere · **Posted:** Tue May 10, 2005 4:07 pm

It can definitely be done:
Something like that:

Code:

<class name="Person" table="person" lazy="true">
        <id name="id" column="id" type="java.lang.Long">
            <generator class="hilo"/>
        </id>        
   <set name="friends" table="person_friends" lazy="true" inverse="true">
         <key column="parent_id"/>
      <many-to-many class="Person" column="child_id"/>
   </set>       
</class>

Hope that helps.

Vincent Giguère

-don · **Joined:** Mon May 09, 2005 5:26 pm **Posts:** 21

Thanks much. I'll pass it along to said developer and give 'er a try.

benethridge · **Posted:** Thu Feb 02, 2006 8:51 am

Sorry, but I don't see how this is a "self-join", since there is a second table, person_friends.

I've been looking through the Hibernate In Action book and the Hibernate Quickly book, and googling, but I don't yet see how to do this.

Does anyone know?

Ben

-don · **Joined:** Mon May 09, 2005 5:26 pm **Posts:** 21

Hi Ben,

It's a self-join in object land for sure -- persons reference persons. Since it's a many-to-many, there *must* be another table as Vincent correctly noted. This isn't a Hibernate thing -- there's just no other way to construct the tables.

To try to use a single table, it'd have to look like:
Person
--------
int personID
String name
int friendID1
int friendID2
..

how many friends would you like to allow?

-don.

benethridge · **Posted:** Sun Apr 09, 2006 5:45 am

An infinite number, as is commonly done.

Ben

bitbyter · **Joined:** Thu Dec 01, 2005 12:23 pm **Posts:** 19

Got also an example how to achieve the same result using Hibernate EJB3 Annotations??

Preferably for a class that got a single parent instance of itself and and a child collection of itself:

Code:

public class Person {

   private String id;

   private Person parent;

   private Collection<Person> childs;
}

Adding such an example to the annotations documentation surely would not be a bad thing. :)

- bitbyter

vgiguere wrote:

It can definitely be done:
Something like that:

Code:

<class name="Person" table="person" lazy="true">
        <id name="id" column="id" type="java.lang.Long">
            <generator class="hilo"/>
        </id>        
   <set name="friends" table="person_friends" lazy="true" inverse="true">
         <key column="parent_id"/>
      <many-to-many class="Person" column="child_id"/>
   </set>       
</class>

Hope that helps.

Vincent Giguère

chandramohan_murkute · **Joined:** Thu May 24, 2007 7:18 am **Posts:** 2

Can you please share the sample code to achive following using annotations.

public class Person {

private String id;

private Person parent;

private Collection<Person> childs;
}

bitbyter wrote:

Got also an example how to achieve the same result using Hibernate EJB3 Annotations??

Preferably for a class that got a single parent instance of itself and and a child collection of itself:

Code:

public class Person {

   private String id;

   private Person parent;

   private Collection<Person> childs;
}

Adding such an example to the annotations documentation surely would not be a bad thing. :)

- bitbyter

vgiguere wrote:

It can definitely be done:
Something like that:

Code:

<class name="Person" table="person" lazy="true">
        <id name="id" column="id" type="java.lang.Long">
            <generator class="hilo"/>
        </id>        
   <set name="friends" table="person_friends" lazy="true" inverse="true">
         <key column="parent_id"/>
      <many-to-many class="Person" column="child_id"/>
   </set>       
</class>

Hope that helps.

Vincent Giguère

bitbyter · **Joined:** Thu Dec 01, 2005 12:23 pm **Posts:** 19

The following code should do what you are looking for!

Code:

@Entity
@Table(name = "TBL_PERSON")
public class Person {

    @Id
    @GeneratedValue(generator = "system-uuid")
    @GenericGenerator(name = "system-uuid", strategy = "uuid")
    @Column(name = ID)
    private String id;

    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "parent")
    private Person parent;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    @JoinColumn(name = "parent")
    @Cascade(value = { org.hibernate.annotations.CascadeType.ALL,
            org.hibernate.annotations.CascadeType.DELETE_ORPHAN })
    private Collection<Person> children = new LinkedList<Person>();
}

HTH

- bitbyter

chandramohan_murkute

chandramohan_murkute wrote:

Can you please share the sample code to achive following using annotations.

public class Person {

private String id;

private Person parent;

private Collection<Person> childs;
}