Many to Many Relation in Hibernate

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Many to Many Relation in Hibernate

There are two table EVENTS and SPEAKERS . One Event can have multiple speakers. And One Speaker can speak in multiple Event. So this is many to many relation. To maintain this relation we have to introduce third TABLE name EVENT_SPEAKERS .

Step 1. create a tables EVENTS , SPEAKERS and EVENT_SPEAKERS in your database

create table EVENTS (
event_id number, //PRIMARY KEY
event_name varchar
);
create table SPEAKERS (
speaker_id number, //PRIMARY KEY
speaker_name varchar,
);
create table EVENT_SPEAKERS (
elt number,//primary key event_id number, speaker_id number
);

Step 2. event.hbm.xml - Hibenate mapping-Which maps to EVENTS TABLE

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="Event" table="events">
<id name="id" column="event_id" type="long">
<generator class="increment"/>
</id>
<property name="name" column="event_name" type="string" length="100"/>
<set name="speakers" table="event_speakers" cascade="all">
<key column="event_id"/>
<many-to-many class="Speaker"/>
</set>
</class>
</hibernate-mapping>

Step 3. Mapping speaker.hbm.xml- Which maps to SPEAKERS TABLE.

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="Speaker" table="speakers">
<id name="id" column="speaker_id" type="long">
<generator class="increment"/>
</id>
<property name="name" column=" speaker_name" type="string" length="100"/>
<set name="events" table="event_speakers" cascade="all">
<key column="speaker_id"/>
<many-to-many class="Event"/>
</set>
</class>
</hibernate-mapping>

Step 4. Create Event.java bean class.


				
				public class Event{

				private long id;

				private String name;

				private Set speakers;

				public void setId(long id) {

				this.id = id;

				}

				public long getId() {

				return id;

				}

				public String getName() {

				return name;

				}

				public void setName(String name) {

				this.name = name;

				}

				public void setSpeakers(Set speakers) {

				this.speakers = speakers;

				}

				public Set getSpeakers() {

				return speakers;

				}

				}

Step 5. Create Speaker.java bean class.


				
				public class Speaker{

				private long id;

				private String name;

				private Set events;

				public void setId(long id) {

				this.id = id;

				}

				public long getId() {

				return id;

				}

				public String getName() {

				return name;

				}

				public void setName(String name) {

				this.name = name;

				}

				public Set getEvents() {

					return this.events;

				}

				public void setEvents(Set events) {

				this.events = events;

				}


				}

Step 6. add event.hbm.xml and speaker.hbm.xml into hibernate.cfg.xml

<?xml version='1.0' encoding='utf-8'?>
<lt;!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>

<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="connection.url">jdbc:mysql://localhost:3306/techfaqdb</property>
<property name="connection.username">techfaq</property>
<property name="connection.password">techfaq</property>

<property name="hibernate.c3p0.min_size">1</property>
<property name="hibernate.c3p0.max_size">4</property>
<property name="hibernate.c3p0.timeout">1800</property>
<property name="hibernate.c3p0.max_statements">50</property>

<property name="dialect">org.hibernate.dialect.MySQLDialect</property>

<property name="current_session_context_class">thread</property>

<property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

<property name="show_sql">true</property>
<property name="hbm2ddl.auto">update</property>
<mapping resource="event.hbm.xml"/>
<mapping resource="speaker.hbm.xml"/>
</session-factory>
</hibernate-configuration>

Here is the code : how to save and fetch data


				
				Event event = new Event();

				event.setName("Inverse test");

				event.setSpeakers(new HashSet());

				event.getSpeakers().add(new Speaker("Ram", event));

				event.getSpeakers().add(new Speaker("Syam", event));

				session.save(event); /// Save All the Data

				event = (Event) session.load(Event.class, event.getId());

				Set speakers = event.getSpeakers();

				for (Iterator i = speakers.iterator(); i.hasNext();) {

				Speaker speaker = (Speaker) i.next();

				System.out.println(speaker.getFirstName());

				System.out.println(speaker.getId());

				}