Unable to locate appropriate constructor error for nested list object in kotlin and JpaRepository

Question

I\'m facing the following error when JPA attempts to map the result from a query to the result repository method DTO:

org.hibernate.hql.internal.ast.QuerySyntaxE         


        

Answer 1

The problem is that database query is able to return plain result only. Persistence provider can convert it to entities with nested entities lists. As for dto you have to solve the problem yourself.

So you can get plain result using User dto with constructor as below

public User(String username, String password, String roleName, String roleDescription) {
     this.username = username;
     this.password = password;
     roles = new ArrayList<>();
     roles.add(new Role(roleName, roleDescription));  
}

Then you need repository method like this

@Query("select new com.example.dto.User(u.username, u.password, r.roleName, r.description) from DbUser u join u.roles r where u.username=:username")
List findUserByUsername(@Param("username") String username);

Handle result in service layer

public Optional findUserByUsername(username) {
    List users = findUserByUsername(username);

    if(users.isEmpty()) {
        return Optional.empty();
    }

    User user = users.get(0);
    if(users.size() > 1) {
         users.subList(1, users.size()).forEach(u -> user.getRoles().addAll(u.getRoles()));
    }

    return Optional.of(user);
}

You can use the same approach for Kotlin