JPA (@OneToMany) query without joining child table

Question

I'm writing a service with JPA and Postgres db. I have a class named Student:

public class Student {
    @id
    private String id;

    private String firstName;
    private String lastName;

    @OneToMany(targetEntity = Phone.class, fetch = FetchType.LAZY, cascade = CascadeType.ALL, orphanRemoval = true)
    @JoinColumn(name = "student_id", referencedColumnName = "id")
    private Set<Phone> phones;

    // Setter and Getter
}

And a Phone class:

public class Phone {
    @id
    private String id;

    private String number;

    // Setter and Getter
}

Now there will be two tables in my db, following are columns in them:

student: id, first_name, last_name

phone: id, number, student_id (generated by @JoinColumn)

Now every time when I query the student, JPA will join the phone table into student table and the student result contains the phone info of this student. This is exactly what I want.

But now I met a problem. When I query the list of the students, the phone info is not useful in this case, only the id, firstName and lastName are necessary. But JPA does the same "join" operation for me too. I know this consumes lots of time. In such case how could I just return the info in the student table? Without joining the phone table?

I tried something in the repository like

@Query(SELECT s.id, s.firstName, s.lastName FROM student s)
public List<Student> findAllStudentWithoutPhone();

but it returns me the list of the values, but not converted into a Student object. How could I achieve this feature?

Answer 1

In the one-to-many mapping (refer below) you have set the fetch type to be lazy fetch = FetchType.LAZY hence hibernate won't fetch set of phones corresponding to the student until you access the set of phones via getter method, so no need to worry about it.

@OneToMany(targetEntity = Phone.class, fetch = FetchType.LAZY, cascade = CascadeType.ALL, orphanRemoval = true)
@JoinColumn(name = "student_id", referencedColumnName = "id")
private Set<Phone>;

However in case you need to verify whether things are working fine or not you can set the property show_sql to true and check the sql generated via hibernate.