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Hibernate One-to-many Relationships
Posted on: January 28, 2010 at 12:00 AM

Here you will learn one-to-many relationships developed using the hmb.xml file.

Hibernate One-to-many Relationships

Hibernate One-to-many Relationships - One to many example code in Hibernate using the xml file as metadata.

Here you will learn one-to-many relationships developed using the hmb.xml file. The one-to-many tag is used to define the relationships.

One-to-Many Relationships

In case of one-to-many relationships, each row in table A linked to multiple rows in table B. Here we will implement one-to-many relationship between two entries Group and Story. There exists multiple stories under a group.

We are using two tables grouptable and story and java entities Group and Story maps respectively. Here is the ER diagram

Java Persistence Objects:

The code for Group.java is given below:




package roseindia;



import java.util.List;





public class Group {

  

  private int id;

  private String name;

  private List stories;



  public Group(){

  }



  public Group(String name) {

  this.name = name;

  }



  public void setId(int i) {

  id = i;

  }



  public int getId() {

  return id;

  }



  public void setName(String n) {

  name = n;

  }



  public String getName() {

  return name;

  }



  public void setStories(List sl) {

  stories = sl;

  }



  public List getStories() {

  return stories;

  }



}

The code for Story.java is given below:


package roseindia;



public class Story {

  

  private int id;

  private String info;

  private Group group;



  public Story(){

  }



  public Group getGroup() {

  return group;

  }



  public void setGroup(Group group) {

  this.group = group;

  }



  public Story(String info) {

  this.info = info;

  }



  public void setId(int i) {

  id = i;

  }



  public int getId() {

  return id;

  }



  public void setInfo(String n) {

  info = n;

  }



  public String getInfo() {

  return info;

  }



}

Mapping xml file (Group.hbm.xml):

This file contains the mapping for both the entities Stroy and Group which maps with the tables gropuptable and story respectively.

<?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>

</id>

<property name="name" type="java.lang.String" column="name"

length="50" />

<one-to-many class="roseindia.Story" />

</list>

</class>

</id>

<property name="info" type="java.lang.String" column="info"

length="255" />

<many-to-one name="group" class="roseindia.Group" column="parent_id" />

</class>

</hibernate-mapping>

The following mapping tag defines the one-to-many mapping between Group and Story entities.

<one-to-many class="roseindia.Story" />

</list>

In Group.java we have stories variable of the list type, so we have used the <list../> tag here. The <one-to-many ../> specifies the one to many relationships to Story entity.

Running the Program:

To run and test the program you have to execute the OneToManyRelation.java. Here is the full code of OneToManyRelation.java file:


package roseindia;



import java.awt.Stroke;

import java.util.ArrayList;

import java.util.Iterator;

import java.util.List;



import org.hibernate.HibernateException;

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.Transaction;

import org.hibernate.cfg.Configuration;



//Goups and Stories



public class OneToManyRelation {



  // Group and Multiple Stories



  public static void main(String[] args) {



  SessionFactory sessFact = null;

  Session session = null;

  try {

  sessFact = new Configuration().configure()

   .buildSessionFactory();

  session = sessFact.openSession();

  Transaction tr = session.beginTransaction();



  // Group Object

  Group group = new Group("Java Group 2");

  // Array list to hold stories

  ArrayList list = new ArrayList();



  // Create Stories

  Story javaHistory = new Story();

  javaHistory.setInfo("Java History");

  list.add(javaHistory);



  // Create Stories

  Story javaPlatform = new Story();

  javaPlatform.setInfo("Java Platforms");

  list.add(javaPlatform);



  // Create Stories

  Story javaNews = new Story();

  javaNews.setInfo("Java News");

  list.add(javaNews);



  // Create Stories

  Story jeeNews = new Story();

  jeeNews.setInfo("JEE News");

  list.add(jeeNews);



  // Add the list to Group object

  group.setStories(list);



  // Save group

  session.save(group);



  // Print the generated group id

  System.out.println("Group id:" + 

  group.getId());



  /*

 * 

 * //Load the group class Group g = (Group)

 * session.load(Group.class, 1); 

   System.out.println("Group Name:" +

 * g.getName());

 * 

 * List listStories = g.getStories();

    Iterator storiesIter =

 * listStories.iterator(); 

   while(storiesIter.hasNext()) { Story

 * story = (Story)storiesIter.next();

 * System.out.println("Story Info:" + 

   story.getInfo()); }

 */



  tr.commit();



  } catch (HibernateException he) {

  System.out.println(he.getMessage());

  } finally {

  // Session close

  session.close();

  // SessionFactory close

  sessFact.close();

  }



  }



}

The above code is self explanatory, we are creating the objects of Group:

// Group Object Group group = new Group("Java Group 2");

Then set the author name:

The create ArrayList object set and add many Story objects:

// Array list to hold stories

ArrayList list = new ArrayList();

// Create Stories

Story javaHistory = new Story();

javaHistory.setInfo("Java History");

list.add(javaHistory);

// Create Stories

Story javaPlatform = new Story();

javaPlatform.setInfo("Java Platforms");

list.add(javaPlatform);

// Create Stories

Story javaNews = new Story();

javaNews.setInfo("Java News");

list.add(javaNews);

// Create Stories

Story jeeNews = new Story();

jeeNews.setInfo("JEE News");

list.add(jeeNews);

Finally add the book set to the group object and save the group object:

// Add the list to Group object

group.setStories(list);

// Save group

session.save(group);

Here is output of the program when executed in Eclipse IDE.

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).

log4j:WARN Please initialize the log4j system properly.

Hibernate: insert into gropuptable (name) values (?)

Hibernate: select max(id) from story

Group id:2

Hibernate: insert into story (info, parent_id, id) values (?, ?, ?)

Hibernate: update story set parent_id=?, idx=? where id=?

In this section we learned about the one-to-many relationships in Hibernate.

In next section we will learn how to create and run the many-to-many mapping in Hibernate. Many to many relationships is interesting as one extra table is needed to persist the relationship.

Download the code example

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