Association is a relationship of one class to another class known as 'relationships' in database terminology. Usually the database design involves the master detail tables to represent the relationship of one entity to another. Struts provides a mechanism to manage one-to-many and many-to-many relationships. Before moving forward,let's create the necessary database tables to show associations.

Let's create a table phone_numbers to represent phone numbers of a user. Following script can be used to create the table in Oracle.

CREATE TABLE PHONE_NUMBERS
(
    USER_ID NUMBER REFERENCES USERS(USER_ID),
    NUMBER_TYPE VARCHAR(50),
    PHONE NUMBER,
    PRIMARY KEY(USER_ID,NUMBER_TYPE)
);

The same table can be created in MySQL using the script given below.

CREATE TABLE PHONE_NUMBERS
(
    USER_ID NUMERIC NOT NULL REFERENCES USERS(USER_ID),
    NUMBER_TYPE CHAR(50) NOT NULL,
    PHONE NUMERIC,
    PRIMARY KEY(USER_ID,NUMBER_TYPE)
);

We have two tables in the database: USERS and the PHONE_NUMBERS with one to many relationship such that one User many contain 0 or more phone numbers. Create a class PhoneNumber and create its mapping file following the steps to create User class. The code for the PhoneNumber class is shown below.

package com.visualbuilder.hibernate;

import java.io.Serializable;

/**

 * PhoneNumber

 * @author VisualBuilder

 */

public class PhoneNumber implements Serializable

{

private long userId = 0;

private String numberType = "home";

private long phone = 0;

public String getNumberType() {

return numberType;

}

public void setNumberType(String numberType) {

this.numberType = numberType;

}

public long getPhone() {

return phone;

}

public void setPhone(long phone) {

this.phone = phone;

}

public long getUserId() {

return userId;

}

public void setUserId(long userId) {

this.userId = userId;

}

}

Note that we have made the PhoneNumber class Serializeable. This is because we need a composite key in this class,and Hibernate requires tat the class that represents the composite key must be Serializeable. The mapping file 'PhoneNumber.hbm.xml' contains following text.

<?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="com.visualbuilder.hibernate.PhoneNumber" table="PHONE_NUMBERS" >
      <composite-id>
            <key-property column="USER_ID"  name="userId" type="java.lang.Long" />
            <key-property column="NUMBER_TYPE"  name="numberType" type="java.lang.String"/>
        </composite-id>
 
        <property name="phone" type="java.lang.Long">
            <column name="PHONE" precision="22" scale="0" />
        </property>
 
  </class>
</hibernate-mapping>
 

Add the mapping resource for PhoneNumber in hibernate.cfg.xml file. To do this,locate the line <mapping resource="com/visualbuilder/hibernate/User.hbm.xml" /> in the hibernate.cfg.xml file and add the following line just after this line.

<mapping resource="com/visualbuilder/hibernate/PhoneNumber.hbm.xml" />

We want to write few lines of code so that when we select a user,we automatically get all the phone numbers associated with this user. To do this,add a method 'getPhoneNumbers' that returns a list of users in class User and 'setPhoneNumbers' that takes a List as argument. The complete listing of class User after adding these two methods is shown below.

package com.visualbuilder.hibernate;

/**

 * @author VisualBuilder

 *

 */

public class User {

private long userId = 0;

private String firstName = "";

private String lastName = "";

private int age = 0;

private String email = "";

private java.util.Set phoneNumbers = new java.util.HashSet();

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

public String getEmail() {

return email;

}

public void setEmail(String email) {

this.email = email;

}

public String getFirstName() {

return firstName;

}

public void setFirstName(String firstName) {

this.firstName = firstName;

}

public String getLastName() {

return lastName;

}

public void setLastName(String lastName) {

this.lastName = lastName;

}

public long getUserId() {

return userId;

}

public void setUserId(long userId) {

this.userId = userId;

}

public java.util.Set getPhoneNumbers() {

return phoneNumbers;

}

public void setPhoneNumbers(java.util.Set phoneNumbers) {

if(phoneNumbers != null)

this.phoneNumbers = phoneNumbers;

}

}

The phoneNumbers HashTable will be used to store all phone numbers associated with this User. The new changes are highlighted in gray. We have added the methods for getting and setting the phone umbers,we need to add the mapping for the phone numbers now. Add the following block in User.hbm.xml before the </class> tag.

<set name="phoneNumbers" cascade="all">

    <key column="USER_ID"/>

    <one-to-many class="com.visualbuilder.hibernate.PhoneNumber" />

</set>

As you can see,the tags are very simple. The cascade attribute of set tells the Hibernate how to deal with the child records when a parent is deleted  or a child is added.

Now it is the time to test our functionality. Remember the TestClient class where we had few blocks of code explaining the save,update and delete functionality. For the time being,comment the call to testDeleteUser and replace the contents of method testUpdateUser with the following code.

  PhoneNumber ph = new PhoneNumber();
  ph.setUserId(user.getUserId());
  ph.setNumberType("Office");
  ph.setPhone(934757);
  user.getPhoneNumbers().add(ph);
 
  ph = new PhoneNumber();
  ph.setUserId(user.getUserId());
  ph.setNumberType("Home");
  ph.setPhone(934757);
  user.getPhoneNumbers().add(ph);

  manager.saveUser(user);
  System.out.println("User updated with ID = " user.getUserId());
 

We are adding two phone numbers to the recently saved user objects. Note that we even don't need to save the PhoneNumber. These are automatically saved as we add them to the User. Query the database and see how these instances are persisted. Once you see the values in the database,re-enable the code to delete the User object and see what happens with the child PhoneNumber objects.

So far,we have dealt with one to many association,we can configure the Hibernate for one to one and many to many associations. For more details on how to make Hibernate configuration for other kind of associations,see http://www.hibernate.org/hib_docs/v3/reference/en/html_single/#tutorial-associations