In my spring boot application, I have an employe entity and a user entity , that are practically the same , just user has email and password so this entity is used only for authentication but this user references a specific employe so in database table user (id,email,password,employe_id) and employe(id,fname,lname,email,password,….,user_id) , and employe can be a manager or HR or just a simple employe so those are the role(id,role_name) and to associate each user with a role there is a join table user_role(user_id,role_id) , so this is my database structure now I am having issues when modifying information of an employee , changing the other fields goes normal but when it comes to changing role,which requires to delete a user then create a user with a different role then associate it with the modified employe :
this is the method that returns that error

 @Transactional
   public void modifyEmployee(Long id, String nom, String prenom, 
  String email, String  motdepass
         Departement departement, Long solde, String role) throws 
     EmployeExistException {
         Employe employe = emp.findById(id)
         .orElseThrow(() -> new NoSuchElementException("Employee not 
  found with ID: " + id));

         // Update employee details
         if (nom != null) employe.setNom(nom);
         if (prenom != null) employe.setPrenom(prenom);
         if (email != null) employe.setEmail(email);
         if (motdepass != null) employe.setMotdepass(motdepass);
         if (departement != null) employe.setDepartement(departement);
         if (solde != null) employe.setSoldeconge(solde);

         // Check if a role change is requested
         if (role != null) {
             User oldUser = employe.getUser();
             if (oldUser != null) {
                 userRepository.delete(oldUser);

                 System.out.println("Deleted old user");
             }

             // Create a new user with the specified role
             User newUser = createUserWithRole(employe, role);
             employe.setUser(newUser);
             System.out.println("Created new user with role: " + role);
         }

         // Save the modified employee
         emp.save(employe);
     }

User :

 @Table(name = "users", 
  uniqueConstraints = { 
    @UniqueConstraint(columnNames = "username"),
    @UniqueConstraint(columnNames = "email") 
})
  public class User {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

@NotBlank
@Size(max = 50)
@Email
 private String email;

 @NotBlank
 @Size(max = 120)
 private String password;

@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(  name = "user_roles", 
    joinColumns = @JoinColumn(name = "user_id"), 
    inverseJoinColumns = @JoinColumn(name = "role_id"))

 private Set<Role> roles = new HashSet<>();
 @OneToOne(mappedBy = "user", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
 private Employe employe;

Employe :

public class Employe {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    protected Long id;
    protected String nom;
    protected String prenom;
    protected String email;
    protected String motdepass;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "departement")
    protected Departement departement;
    protected Long soldeconge;
    @OneToMany(mappedBy = "emp", cascade = CascadeType.ALL)
    protected List<Demande> demandes;
    @OneToMany(mappedBy = "emp", cascade = CascadeType.ALL)
    protected List<Conge> conges;
    protected LocalDate created_at;
    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    private User user;