Hibenate "Parameter value did not match expected type"

I want to insert a List<Completed> objects into a relational table, using Spring framework and Hibernate.

My entity class has the Collections List field -

@Entity
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    ...
    
    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
    private List<Completed> completedList;

    ... // getters/setters
}

The data access repository -

public interface UserRepository extends CrudRepository<User, Long> {

    Optional<User> findByEmail(String email);

    @Modifying
    @Query(value = "update User set completedList = :completed where id = :id")
    @Transactional
    void addNewCompletion(@Param("completed") List<Completed> completed,
                          @Param("id") long userID);
}

I'm calling the method as such (for testing purposes) -

        Completed completed1 = new Completed(1, LocalDateTime.now());
        Completed completed2 = new Completed(2, LocalDateTime.now());
        Completed completed3 = new Completed(3, LocalDateTime.now());
        List<Completed> completedList = List.of(completed1, completed2, completed3);
//        completedList.add(completed1);
        UserService.updateUserCompletion(completedList, UserService.findUserID(currentUser));

I have also tried -

    List<Completed> completedList = new ArrayList()<>;
    completedList.add(new Completed()); // etc.

   Completed completed = new Completed(1, LocalDateTime.now());
   UserService.updateUserCompletion(completed, UserService.findUserID(currentUser));

Either which way results in the same exception -

java.lang.IllegalArgumentException: Parameter value [io.github.siaust.web_quiz_app.Model.Completed@2f1eafc2] did not match expected type [java.util.Collection (n/a)]

I can't understand as I am passing a List<Completed> object to the @Param of the custom @Query? Why does the type not match?

It's not possible to update database relations like this. Collections is a specific type and giving a list of object in that way you give Hibernate no change to recognize this as a list and handle it correctly. When you use @Query hibernate simply runs your SQL code. That means Hibernate passes into your SQL code a collection as a param and database server could not resolve that collection You also creating an immutable collection which is don't work here too. I think in your case this should work correctly:

User user = UserService.findUserID(currentUser);

List<Completed> completedList = new ArrayList<>();
completedList.add(new Completed(1, LocalDateTime.now()));
completedList.add(new Completed(2, LocalDateTime.now()));
completedList.add(new Completed(3, LocalDateTime.now()));


user.setCompleted(completedList);
userRepository.save(user);