hibernate Association Mappings between Entities OneToMany association


Example

To illustrate relation OneToMany we need 2 Entities e.g. Country and City. One Country has multiple Cities.

In the CountryEntity beloww we define set of cities for Country.

@Entity
@Table(name = "Country")
public class CountryEntity implements Serializable
{
   private static final long serialVersionUID = 1L;
 
   @Id
   @Column(name = "COUNTRY_ID", unique = true, nullable = false)
   @GeneratedValue(strategy = GenerationType.SEQUENCE)
   private Integer           countryId;
 
   @Column(name = "COUNTRY_NAME", unique = true, nullable = false, length = 100)
   private String            countryName;

   @OneToMany(mappedBy="country", fetch=FetchType.LAZY)
   private Set<CityEntity> cities = new HashSet<>();

   //Getters and Setters are not shown
}

Now the city entity.

@Entity
@Table(name = "City")
public class CityEntity implements Serializable
{
   private static final long serialVersionUID = 1L;
 
   @Id
   @Column(name = "CITY_ID", unique = true, nullable = false)
   @GeneratedValue(strategy = GenerationType.SEQUENCE)
   private Integer           cityId;
 
   @Column(name = "CITY_NAME", unique = false, nullable = false, length = 100)
   private String            cityName;

   @ManyToOne(optional=false, fetch=FetchType.EAGER)
   @JoinColumn(name="COUNTRY_ID", nullable=false)
   private CountryEntity country;
 
   //Getters and Setters are not shown
}