How to Map One to Many Correctly in Hibernate

Question

Good afternoon, I am creating a very simple Restful API that implements an online course system, where I have only 2 tables "Courses" and "Teachers" (And I am trying to learn Latin in the process):

I managed to make everything work right until I need to do an Inner Join between the 2 tables, now I can’t return the values... I wanted something like:

Teacher class

@Entity
public class Teacher {

    @Id
    @Column(name="teacher_id")
    private int id;
    @Column(name="teacher_name")
    private String name;
    @Column(name="teacher_phone")
    private String phoneNumber;
    private List<Course> courses;

    public Teacher() {
        courses = new ArrayList<>();
    }

    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 getPhoneNumber() {
        return phoneNumber;
    }

    public void setPhoneNumber(String phoneNumber) {
        this.phoneNumber = phoneNumber;
    }

    @OneToMany(mappedBy="teacher")
    public List<Course> getCourses() {
        return courses;
    }

 }

Course class

@Entity
public class Course {

    @Id
    @Column(name="course_id")
    private int id;
    @Column(name="course_name")
    private String name;
    @Column(name="course_schedule")
    private String schedule;


    private Teacher teacher;

    public Course() {

    }

    @ManyToOne(cascade = CascadeType.PERSIST)
    @JoinColumn(name="teacher_id")
    public Teacher getTeacher() {
        return teacher;
    }


    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    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 getSchedule() {
        return schedule;
    }

    public void setSchedule(String schedule) {
        this.schedule = schedule;
    }
   }

Database Access Class that would query

public interface CoursesRepository extends CrudRepository<Course, Integer>{


@Query("select c.course_id, c.course_name, c.course_schedule, t.teacher_name "
        + "from course c "
        + "inner join teacher t "
        + "on c.teacher_id = t.teacher_id")
public List<Course> findAllCoursesAndTeachers();

}

I’m getting an error that Spring can’t create the bean for my class, but I think it’s a problem with my relationships and/or my query (In the example query I showed I only had the name of the teacher, but I would have no problem if I brought the whole object) :

Caused by: org.hibernate.MappingException: Could not determine type for: java.util.List, at table: teacher, for columns: [org.hibernate.mapping.Column(courses)]

That means I shouldn’t have the course list in the teacher class?

Thanks in advance.

Answer 1

Put the entities relationship in the statement:

TEACHER CLASS

@Entity
public class Teacher {

    @Id
    @Column(name="teacher_id")
    private int id;
    @Column(name="teacher_name")
    private String name;
    @Column(name="teacher_phone")
    private String phoneNumber;
    @OneToMany(mappedBy="teacher")
    private List<Course> courses;

    public Teacher() {
        courses = new ArrayList<>();
    }

    // GETTERS AND SETTERS

}

COURSE CLASS

@Entity
public class Course {

    @Id
    @Column(name="course_id")
    private int id;
    @Column(name="course_name")
    private String name;
    @Column(name="course_schedule")
    private String schedule;
    @ManyToOne
    @JoinColumn(name = "teacher_id")
    private Teacher teacher;

    public Course() {

    }

    // GETTERS AND SETTERS

}

For more information on mapping, take a look at this response: /a/234768/132963