One-to-One mapping can be done in 2 ways:

1. Primary key association:
2. Foreign key association.

My previous post described about the Primary key one-to-one mapping in Hibernate. This post gives an example of Foreign key one-to-one association in Hibernate.

In Foreign key one-to-one mapping,  2 entities will have one to one association using foreign keys. one  table holds primary key of 2nd table. Eg: consider Customer and address entities. In primary key association,  both the tables share same primary key. In foreign key association, customer table will have a address_id columns, in which primary key (address_id) of the customer’s address will be saved. Both customer and address tables will have different primary keys.

The tables would look like below:

create table address(
    address_id int(11) not null auto_increment,
    address_line1 varchar(100) not null,
    address_line2 varchar(100),
    city varchar(50) not null,
    state varchar(50) not null,
    pincode int(10) not null,
    primary key(address_id));

create table customer(
    customer_id int(11) not null auto_increment,
    name varchar(50) not null,
    email_id varchar(100),
    contact_no varchar(20),
    address_id int(11),
    primary key(customer_id),
    foreign key(address_id) references address(address_id));

Address is a simple table and customer table contains a column to store the corresponding address_id.

Corresponding Beans would be with following fields and their getter and setter methods:

public class Address {

 long addressId;
 String addressLine1;
 String addressLine2;
 String city;
 String state;
 Integer pincode;
 Customer customer;

public class Customer {
 Long customerId;
    String name;
    String emailAddress;
    String contactNo;
    Address address;

The Hibernate mapping files would look like below:

Address.hbm.xml:

<?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=”entities.Address” table=”ADDRESS”>
    <id name=”addressId” type=”long” column=”address_id” >
     <generator/>
  </id>
  
  <property name=”addressLine1″>
     <column name=”address_line1″ />
    </property>
   
    <property name=”addressLine2″>
     <column name=”address_line2″ />
    </property>
   
    <property name=”city”>
     <column name=”city” />
    </property>
   
    <property name=”state”>
     <column name=”state” />
    </property>
   
    <property name=”pincode”>
     <column name=”pincode” />
    </property>
  </class>
</hibernate-mapping>

address.hbm.xml is simple and there is no much complications.

Customer.hbm.xml:

<?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=”entities.Customer” table=”CUSTOMER”>
    <id name=”customerId” type=”long” column=”customer_id” >
     <generator/>
  </id>
  
  <property name=”name”>
     <column name=”NAME” />
    </property>
   
    <property name=”emailAddress”>
     <column name=”email_id” />
    </property>
   
    <property name=”contactNo”>
     <column name=”contact_no” />
    </property>

    <many-to-one name=”address”
                column=”address_id”
                cascade=”all”  unique=”true”
                />
  </class>
</hibernate-mapping>

Point to note here is the Many-to-one tag. We are associating the entities with one-to-one but we are using a many-to-one tag. When using many-to-one in one-to-one mapping, we ignore the ‘many’ part. We are not caring how many address are bound to a customer, but here we ensure that only one customer (to-one part) is bound to an address.

In customer class, by defining the address variable as below:

 Address address;

 (As its not a List/Set or collection) we ensure that its just an one-to-one relationship.

 Now we can test it using following code:

// save customer
         Customer customer = new Customer();
         customer.setName(“Surya”);
         customer.setEmailAddress(“surya@gmail.com“);
         customer.setContactNo(“91-932686876″);
         Address address = new Address();
         address.setAddressLine1(“xxx-street, near Surya Complex”);
         address.setCity(“Pune”);
         address.setState(“Maharashtra”);
         address.setPincode(11111);
        
         customer.setAddress(address);
         address.setCustomer(customer);
         session.save(customer);

   //fetch all customers
           
            Query query = session.createQuery(“from Customer customer”);
            for(Iterator it=query.iterate();it.hasNext();){
             Customer customer1 = (Customer) it.next();
             Address address1 = customer1.getAddress();
                System.out.println(“customer ID: ” + customer1.getCustomerId());
                System.out.println(“Name: ” + customer1.getName());
                System.out.println(“address: ” + address1.getAddressLine1());
                System.out.println(address1.getAddressId());
                System.out.println(address1.getAddressLine2());
                System.out.println(address1.getCity());
                System.out.println(address1.getState());
                System.out.println(address1.getPincode());
              }

To download full code of above example please click here.