One-To-One Association

Let's extend our model of the world to include heads of state.
HEAD_OF_STATE and COUNTRY tables

This is the data in the HEAD_OF_STATE table:

HOS_ID	CTRY_ID	HOS_NAME
1	122	President Mikheil Saakashvili
2	14	President Horst Köhler
3	73	President Karolos Papoulias

We map the HeadOfState class to the table and include a @OneToOne association with the Country class.

@Entity
@Table(name="HEAD_OF_STATE")
public class HeadOfState {

  @Id
  @GeneratedValue
  @Column(name="HOS_ID")
  private Integer id;

  @Column(name="HOS_NAME")
  private String name;

  @OneToOne
  @JoinColumn(name="CTRY_ID")
  private Country country;

  // Accessors

}

The CTRY_ID column, which is specified in the @JoinColumn annotation, names the foreign key column on the HEAD_OF_STATE table which is used to join the two tables together.

We make the association bidirectional by adding a reference to HeadOfState on the Country class.

@OneToOne (mappedBy="country")
@JoinColumn(name = "CTRY_ID")
private HeadOfState headOfState;

Remember to add mappedBy on the @OneToOne annotation to ensure that only one side manages the relationship.

We can now access the head of state for a country

Country georgia = (Country) session.load(Country.class, 122);
System.out.println("The head of state for " + georgia.getName() +
             " is " + georgia.getHeadOfState().getName());

or the country represented by a head of state.

HeadOfState hos = (HeadOfState) session.load(HeadOfState.class, 1);
System.out.println(hos.getName() + " is head of state for " +
             hos.getCountry().getName());

many to many