Many To One Mapping

Parameters#

Column	Column
@TableGenerator	Uses table generator strategy for automatic id creation
@GeneratedValue	Specifies that the value applied to fields is a generated value
@Id	Annotates the field as identifier
@ManyToOne	Specifies Many to One relationship between Employee and Department. This annotation is marked on many side. i.e. Multiple employees belong to a single department. So Department is annotated with @ManyToOne in Employee entity.
@JoinColumn	Specifies database table column which stores foreign key for related entity
## Employee to Department ManyToOne relationship
Employee Entity

@Entity
public class Employee {

    @TableGenerator(name = "employee_gen", table = "id_gen", pkColumnName = "gen_name", valueColumnName = "gen_val", allocationSize = 1)
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "employee_gen")
    private int idemployee;
    private String firstname;
    private String lastname;
    private String email;

    @ManyToOne
    @JoinColumn(name = "iddepartment")
    private Department department;

    // getters and setters
    // toString implementation
}

Department Entity

@Entity
public class Department {

    @Id
    private int iddepartment;
    private String name;

    // getters, setters and toString()
}

Test class

public class Test {

    public static void main(String[] args) {

        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("JPAExamples");
        EntityManager em = emf.createEntityManager();
        EntityTransaction txn = em.getTransaction();

        Employee employee = new Employee();
        employee.setEmail("someMail@gmail.com");
        employee.setFirstname("Prasad");
        employee.setLastname("kharkar");

        txn.begin();
        Department department = em.find(Department.class, 1);//returns the department named vert
        System.out.println(department);
        txn.commit();

        employee.setDepartment(department);

        txn.begin();
        em.persist(employee);
        txn.commit();

    }

}