Hibernate One-to-many Relationships

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Hibernate One-to-many Relationships

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

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>

<class name="roseindia.Group" table="gropuptable">

<id name="id" type="int" column="id">

<generator class="native" />

</id>

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

length="50" />

<list name="stories" cascade="all">

<key column="parent_id" />

<index column="idx" />

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

</list>

</class>

<class name="roseindia.Story" table="story">

<id name="id" type="int" column="id">

<generator class="increment" />

</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.

<list name="stories" cascade="all">

<key column="parent_id" />

<index column="idx" />

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

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

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

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

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

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

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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