Hibernate mapping explained:

One-to-Many type of mapping
example:

Case study: A Department can have many employees and an
Employee can be assigned to one Department.

So Employee and Department are having one to many type of
mapping, is explained here with a one-to-many type of mapping
in Hibernate.

 

So Department and Employees can share a one-to-many type of mapping.


My Software environment for this example:
1. Eclipse 3.2
2. JDK 1.5
3. Hibernate 3.2

Hibernate one to many mapping can be realized by using specific tags
like <one-to-many>, but I am using it along with <set> tag.

In this example, POJO (Plain Old Java Object) are Dept and Employee.

/**
*  This source is provided as is, without any warranty
*  and /or guaranty of any kind.
*  Copyright (C) 2008, iSHTIAK, All Rights Reserved.
*  You can use it for Personal Learning purpose only.
*  E-mail: usingframeworks@gmail.com
*/

Dept.java
package demo;

import java.util.Set;

public class Dept {
	private int deptId;
	private String deptName;
	private Set employees;
	public int getDeptId() {
		return deptId;
	}
	public void setDeptId(int deptId) {
		this.deptId = deptId;
	}
	public String getDeptName() {
		return deptName;
	}
	public void setDeptName(String deptName) {
		this.deptName = deptName;
	}
	public Set getEmployees() {
		return employees;
	}
	public void setEmployees(Set employees) {
		this.employees = employees;
	}
}



Dept is a simple POJO class with a employees field declared
as Set for storing all employees in it.



/**
*  This source is provided as is, without any warranty
*  and /or guaranty of any kind.
*  Copyright (C) 2008, iSHTIAK, All Rights Reserved.
*  You can use it for Personal Learning purpose only.
*  E-mail: usingframeworks@gmail.com
*/

Employee.java
package demo;

public class Employee {
	private int employeeId;
	private String employeeName;
	private Dept dept;
	public Dept getDept() {
		return dept;
	}
	public void setDept(Dept dept) {
		this.dept = dept;
	}
	public int getEmployeeId() {
		return employeeId;
	}
	public void setEmployeeId(int employeeId) {
		this.employeeId = employeeId;
	}
	public String getEmployeeName() {
		return employeeName;
	}
	public void setEmployeeName(String employeeName) {
		this.employeeName = employeeName;
	}
}


HBM mapping file is as mentioned below:

In order to create this mapping, first of all I placed normal <class>
tag for Dept and Employee, with a id for the primary key and property tags
for each fields within Dept and Employee.

In order to use <one-to-many> tag , one should have a collection
of many items at one side, and we can see employees field in Dept.

So there has to have a collection tag like <set> tag inside <class>
tag for Dept.


<?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 package="demo">
  <class name="Dept" table="dept">
    <id name="deptId" column="dept_id">
      <generator class="assigned"/>
    </id>
    <property name="deptName" column="dept_name" type="java.lang.String"/>
    <set name="employees" table="employee" cascade="persist,delete">
      <key column="dept_id" />
      <one-to-many class="Employee"/>
    </set>
  </class>
  <class name="Employee" table="employee">
    <id name="employeeId" column="employee_id">
      <generator class="assigned"/>
    </id>
    <property name="employeeName" column="employee_name" type="java.lang.String"/>
    <one-to-one name="dept" class="Dept" />
  </class>
</hibernate-mapping>


Corresponding tables in database (for this example, I have used HSQLDB), following
are the SQL (for HSQLDB as database):

 create table dept
 (dept_id integer, dept_name varchar(20), primary key (dept_id))

 create table employee
 (employee_id integer, employee_name varchar(50), dept_id integer,
 primary key (employee_id), foreign key(dept_id) references dept(dept_id));


hibernate.cfg.xml file that contains all the configuration settings for this
example is as shown below:

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
	<session-factory>
		<!-- properties -->
		<property name="connection.driver_class">org.hsqldb.jdbcDriver</property>
		<property name="connection.url">jdbc:hsqldb:hsql://localhost/</property>
		<property name="connection.username">sa</property>
		<property name="connection.password"></property>
		<property name="dialect">org.hibernate.dialect.HSQLDialect</property>
		<property name="show_sql">true</property>
		<property name="current_session_context_class">thread</property>
		<!-- mapping files -->
		<mapping resource="demo/DeptsEmployee.hbm.xml"/>
	</session-factory>
</hibernate-configuration>


Client code for testing this example could be something like as follows:
If wanted to have complete source code of this Client program, then
Please REFER THIS


    if(sessionFactory != null) {
	Session session = sessionFactory.getCurrentSession();
	Transaction trans = session.getTransaction();
	trans.begin();
	Employee emp1 = new Employee();
	emp1.setEmployeeId(1001);
	emp1.setEmployeeName("test1");

	Employee emp2 = new Employee();
	emp2.setEmployeeId(1002);
	emp2.setEmployeeName("test2");
	Employee emp3 = new Employee();
	emp3.setEmployeeId(1003);
	emp3.setEmployeeName("test3");

	Set emps = new HashSet();
	emps.add(emp1);
	emps.add(emp2);
	emps.add(emp3);
	Dept dept = new Dept();
	dept.setDeptId(1001);
	dept.setDeptName("test dept1");
	dept.setEmployees(emps);
	session.persist(dept);
	trans.commit();
      }


Here in this client, I am just creating three employees and then
creating a dept and associating these already created employees to dept.

This way, I am getting a following console as output:


Hibernate: insert into dept (dept_name, dept_id) values (?, ?)
Hibernate: insert into employee (employee_name, employee_id) values (?, ?)
Hibernate: insert into employee (employee_name, employee_id) values (?, ?)
Hibernate: insert into employee (employee_name, employee_id) values (?, ?)
Hibernate: update employee set dept_id=? where employee_id=?
Hibernate: update employee set dept_id=? where employee_id=?
Hibernate: update employee set dept_id=? where employee_id=?




This means I have managed to create a one to many mapping persisted in
database tables.



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