Hibernate - Nie mogę ogarnąc prostej relacji one to one.

package domain;

import javax.persistence.*;
import java.lang.reflect.Type;

@Entity
@Table(name = "contact")
public class Contact {
    @Id
    @GeneratedValue
    private int id;
    private String name;
    private String email;

    @OneToOne(mappedBy = "contact", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private Group group;

    public Contact(){}

    public Contact(String name, String email) {
        this.name = name;
        this.email = email;
    }

    @Override
    public String toString() {
        return "Contact{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", email='" + email + '\'' +
                ", group=" + group +
                '}';
    }

    // Getters and Setters

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public Group getGroup() {
        return group;
    }

    public void setGroup(Group group) {
        this.group = group;
    }
}

package domain;

import javax.persistence.*;

@Entity
@Table(name = "group")
public class Group {

    @Id
    //@GeneratedValue
    private int groupId;

    private String groupName;

    @OneToOne
    @MapsId
    private Contact contact;

    // Constructors

    public Group(){}

    public Group(String groupName) {
        this.groupName = groupName;
    }

    // Getters and Setters


    public int getGroupId() {
        return groupId;
    }

    public void setGroupId(int groupId) {
        this.groupId = groupId;
    }

    public String getGroupName() {
        return groupName;
    }

    public void setGroupName(String groupName) {
        this.groupName = groupName;
    }

    public Contact getContact() {
        return contact;
    }

    public void setContact(Contact contact) {
        this.contact = contact;
    }
}

package domain;

import hibernate.utils.HibernateUtils;
import org.hibernate.Session;

import java.util.List;

public class ContactRepository {

    public static void listAllContacts(){

        Session session = null;
        try {
            session = HibernateUtils.openSession();
            List<Contact> listEmployee = session.createQuery("SELECT c FROM Contact c").getResultList();

            if (listEmployee == null) {
                System.out.println("No employee found . ");
            } else {
                for (Contact contact : listEmployee) {
                    System.out.println(contact.toString());
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            if (session != null && session.isOpen()) {
                session.close();
            }
        }


    }

    public static void listContactById(Integer id){
        Session session = null;
        try {
            session = HibernateUtils.openSession();
            try{
                Contact contactById = session.find(Contact.class, id);
                System.out.println(contactById);
            } catch (NullPointerException e){
                System.out.println("No contact with id: " + id);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            if (session != null && session.isOpen()) {
                session.close();
            }
        }
    }

    public static void addNewContact(Contact contact){

        Session session = null;
        try {
            session = HibernateUtils.openSession();
            session.getTransaction().begin();
            session.saveOrUpdate(contact);
            session.getTransaction().commit();
        }catch (Exception e) {
            e.printStackTrace();
        }finally {
            if (session != null && session.isOpen()) {
                session.close();
            }
        }
    }

    public static void editExistingContact(Contact contactToUpdate){

        Session session = null;
        try {
            session = HibernateUtils.openSession();
            session.getTransaction().begin();
            session.merge(contactToUpdate);
            session.getTransaction().commit();
        }catch (Exception e){
            e.printStackTrace();
        }finally{
            if (session != null && session.isOpen()) {
                session.close();
            }
        }

    }

    public static void deleteContact(int idToDelete){

        Session session = null;
        try {
            session = HibernateUtils.openSession();

            Contact contactToDelete = session.find(Contact.class, idToDelete);
            System.out.println(contactToDelete);

            session.getTransaction().begin();
            session.remove(contactToDelete);
            session.getTransaction().commit();

        }catch (Exception e){
            e.printStackTrace();
        }finally{
            if (session != null && session.isOpen()) {
                session.close();
            }
        }
    }
}

package domain;

import hibernate.utils.HibernateUtils;
import org.hibernate.Session;

public class GroupRepository {

    public static void addNewGroup(Group group){

        Session session = null;
        try {
            session = HibernateUtils.openSession();
            session.getTransaction().begin();
            session.saveOrUpdate(group);
            session.getTransaction().commit();
        }catch (Exception e) {
            e.printStackTrace();
        }finally {
            if (session != null && session.isOpen()) {
                session.close();
            }
        }
    }
}

import domain.Contact;
import domain.ContactRepository;
import domain.Group;
import domain.GroupRepository;
import hibernate.utils.HibernateUtils;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

import java.util.Scanner;

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

        Scanner scanner = new Scanner(System.in);
        boolean run = true;

        do {

            // MENU
            System.out.println("");
            System.out.println("MENU");
            System.out.println("=====================");
            System.out.println("1. List all contacts");
            System.out.println("2. Get contact by id");
            System.out.println("3. Add new contact");
            System.out.println("4. Update contact");
            System.out.println("5. Delete contact");
            System.out.println("6. Exit");
            System.out.println("=====================");
            System.out.println("");

            int choice = scanner.nextInt();

            switch (choice){

                case 1:
                    ContactRepository.listAllContacts();
                    break;
                case 2:
                    System.out.println("Type employee id:");
                    int id = scanner.nextInt();
                    ContactRepository.listContactById(id);
                    break;
                case 3:
                    System.out.println("Name: ");
                    String name = scanner.next();
                    System.out.println("Email: ");
                    String email = scanner.next();
                    System.out.println("Group: ");
                    String groupName = scanner.next();
                    Contact contact = new Contact(name, email);
                    Group group = new Group(groupName);
                    contact.setGroup(group);
                    ContactRepository.addNewContact(contact);
                    GroupRepository.addNewGroup(group);
                    break;