Associations and Joins

This section includes a brief introduction about Associations and Joins along with examples.

Association mappings: Association mappings comes in the list of the most difficult thing to get right. This section includes the canonical cases one by one. Here we starts the section from unidirectional mappings and comes to the bi-directional cases.

We'll include the classification of associations that whether they map or not to an intervening join table and by multiplicity.

Here we are not using the Nullable foreign keys since these keys do not require good practice in traditional data modeling. Hibernate does not require the foreign keys since mappings work even if we drop the nullability constraints.

Hibernate HQL Inner Join

Hibernate's HQL language is not capable for handling the "inner join on" clauses. If our domain entity model includes relationships defined between the related objects then the query like this

Query query = session.createQuery("from Car car inner join Owner owner where owner.Name ='Vinod'");

will work. HQL keeps the information of joining the Car and Owner classes according to the association mapping contained in the hbm.xml file. Since the association information is defined in the mapping file so there is no need to stipulate the join in the query. In the above query "from Car car where car.Owner.Name='Vinod'" will work too. Initialization of the collections and many-to-one mapping requires the explicit joins just to avoid lazy load errors.

A unidirectional one-to-many association on a foreign key is rarely required.


<?xml version="1.0" encoding="UTF-8"?>



 <!DOCTYPE hibernate-mapping

   PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"

   "http://hibernate.sourceforge.net/

hibernate-mapping-3.0.dtd">



<hibernate-mapping>

   <class name="net.roseindia.Dealer">

      <id name="id" type="int">

         <generator class="increment"/>

      </id>



      <property name="name" type="string"/>

      <bag name="product" inverse="

true" cascade="all,delete-orphan">

        <key column="did"/>

        <one-to-many class="

net.roseindia.Product"/>

      </bag>



   </class>

</hibernate-mapping>

A unidirectional one-to-many association on a foreign key is rarely required.


<?xml version='1.0' encoding='utf-8'?>

<!DOCTYPE hibernate-mapping

   PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"

   "http://hibernate.sourceforge.net/

hibernate-mapping-3.0.dtd">



<hibernate-mapping>

   <class name="net.roseindia.Product">

      <id name="id" type="int">

         <generator class="increment"/>

      </id>



      <property name="name" type="string"/>

      <property name="did" type="int"/>

      <property name="price" type="double"/>

      <many-to-one name="dealer" class="

net.roseindia.Dealer" column="did" 

insert="false" update="false"/>



   </class>

 

</hibernate-mapping>

Here is the hibernate code:

In this example first we create the session object with the help of the SessionFactory interface. Then we use the createQuery() method of the Session object which returns a Query object. Now we use the openSession() method of the SessionFactory interface simply to instantiate the Session object. And the we retrieve the data from the database store it in that Query object and iterate this object with the help of Iterator and finally displays the requested data on the console.


package net.roseindia;



import java.util.Iterator;

import java.util.List;



import org.hibernate.Query;

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.cfg.Configuration;



public class Join {



  /**

   * @param args

   */

  public static void main(String[] args) {

    // TODO Auto-generated method stub

    Session sess = null;

    try{

      SessionFactory fact = new 

Configuration().configure().buildSessionFactory();

      sess = fact.openSession();

      String sql_query = "from 

Product p inner join p.dealer as d";

      Query query = sess.createQuery(sql_query);

      Iterator ite = query.list().iterator();

      System.out.println("Dealer 

Name\t"+"Product Name\t"+"Price");

      while ( ite.hasNext() ) {

          Object[] pair = (Object[]) ite.next();

          Product pro = (Product) pair[0];

          Dealer dea = (Dealer) pair[1];

          System.out.print(pro.getName());

          System.out.print("\t"+dea.getName());

          System.out.print("\t\t"+pro.getPrice());

          System.out.println();

      }

      sess.close();

    }

    catch(Exception e ){

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

    }



  }



}

Output:

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

log4j:WARN Please initialize the log4j system properly.

Hibernate: select product0_.id as id1_0_, dealer1_.id as id0_1_, product0_.name as name1_0_, product0_.did as did1_0_, product0_.price as price1_0_, dealer1_.name as name0_1_ from Product product0_ inner join Dealer dealer1_ on product0_.did=dealer1_.id

Dealer Name Product Name Price

Computer Agrawal 23000.0

Keyboard Agrawal 1500.0

Computer Agrawal 100.0

Mobile Mohan 15000.0

PenDrive Mohan 200.0

Laptop Ritu 200.0

HardDisk Ritu 2500.0