One to many mapping

One-to-many mapping means a mapping in which each record in one table is linked to multiple records in another table. Like a parent record in one table can have several child records in another table.The parent is not required to have child records i.e allows zero child records, a single child record or multiple child records. The important thing is that the child cannot have more than one parent record.

This can be explained with a simple example, lets have a look on this:

There are two persistent classes Customer.java & Order.java

Customer.java

public class Customer implements java.io.Serializable {
    private long customerId;
    private String firstName;
    private String lastName;
    private Set orders = new HashSet();

    public Customer() {
    };

public class Order {
    private Long orderId;
    private Long quantity;
    private String orderDescription;
    private Date orderDate;

    private Customer customer;

There are two mapping files for the persistent classes namely, customer.hbm.xml & order.hbm.xml

Mapping file is used to define customer table attributes.

customer.hbm.xml

<hibernate-mapping>
 <class name="com.hibernateExample.Customer" table="customer">
<id name="customerId" type="java.lang.Long">
            <column name="customerId">
            <generator class="identity">
        </generator></column></id>
        <property name="firstName" type="string">
            <column name="firstName" length="20" not-null="true">
        </column></property>
        <property name="lastName" type="string">
            <column name="lastName" length="20" not-null="true">
        </column></property>
        <set name="orders" table="order" inverse="true" lazy="true" fetch="select">
            <key><column name="customerId" not-null="true"></column></key>
            <one-to-many class="com.hibernateExample.Order">
        </one-to-many></set>
    </class>
</hibernate-mapping>

This mapping file is used to define order table attribute

order.hbm.xml

<hibernate-mapping>
    <class name="com.hibernateExample.Order" table="order">
        <id name="orderId" type="java.lang.Long">
            <column name="orderId">
            <generator class="identity">               
        </generator></column></id>
        <many-to-one name="customer" class="com.hibernateExample.Customer" fetch="select">
            <column name="customerId" not-null="true"></column>    
        </many-to-one>
        <property name="quantity" type="java.lang.Long">
            <column name="quantity" length="10" not-null="true"></column>
        </property>
        <property name="orderDescription" type="string">
            <column name="orderDescription" length="50" not-null="true"></column>
        </property>
        <property name="orderDate" type="date">
            <column name="orderDate" length="10" not-null="true"></column>
        </property>
    </class>
</hibernate-mapping>

A configuration file named hibernate.cfg.xml contains information about the database and mapping file.

<hibernate-configuration>
    <session-factory>
        <property name="connection.url">jdbc:mysql://localhost:3306/hibernateExample</property>
        <property name="connection.username">root</property>
        <property name="connection.password"></property>
        <property name="connection.driver&#95;class">com.mysql.jdbc.Driver</property>
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>

        <mapping resource="com/hibernateExample/customer.hbm.xml"></mapping>
        <mapping resource="com/hibernateExample/order.hbm.xml"></mapping>       
    </session-factory>
</hibernate-configuration>

Now lets create App.java class, which is going to create two tables in database as, customer & order and are linked with one-to-many relationship.

public class App {
    public static void main(String[] args) {
    ...................................
            .................................
             ................................

     Customer customer = new Customer();
        customer.setFirstName("Amit");
        customer.setLastName("Singh");

        session.save(customer);

        Set orders = new HashSet();

        Order dellLaptops = new Order();
        dellLaptops.setCustomer(customer);
        dellLaptops.setOrderDate(new Date());
        dellLaptops.setQuantity(30l);
        dellLaptops.setOrderDescription("30 Dell latitude D830 laptops");

        Order sonyLaptops = new Order();
        sonyLaptops.setCustomer(customer);
        sonyLaptops.setOrderDate(new Date());
        sonyLaptops.setQuantity(40l);
        sonyLaptops.setOrderDescription("40 sony viao laptops");

        orders.add(dellLaptops);
        orders.add(sonyLaptops);
        customer.setOrders(orders);

                .................................
                ..............................
                .........................

    }
}

Hope this help someone!