It seems I am having a difficult time understanding JPA and how the OneToMany relationship actually works.
For example, imagine I have an object Class
@Entity
public class Class {
@Id
private String className;
@OneToMany(cascade = Cascade.ALL, orphanRemoval = true)
private Set<Student> students;
// Constructors, Getters, Setter
}
I also have an object Student
where it holds Class.
@Entity
public class Student {
@Id
private String studentName;
@ManyToOne
private Class class;
// Constructors, Getters, Setter
}
Obviously, a student can have multiple classes but forget about that.
Why is that when I build a class and then build a couple students using that class, findAll()
on the ClassRepository
returns me an empty set of students.
Class class = new Class("CS", new HashSet<>());
classRepository.save(class); // repository has no special functions
Student student1 = new Student("1", class);
Student student2 = new Student("2", class);
studentRepository.save(student1);
studentRepository.save(student2);
classRepository.findAll() // Returns me a List<Class> with only one class object that has an empty set.
I was thinking the above code should automatically see that the two students are from that one class and so when I call buildingRepository.findAll()
, it will return a Class
object with the students set populated properly.
Is my understanding wrong then? Or is my code wrong? And how can I change it up to fix it?
You can choose:
1. Unidirectional @OneToMany
:
@Entity
public class Class {
@Id
private String className;
@OneToMany(cascade = Cascade.ALL, orphanRemoval = true)
private List<Student> students=new ArrayList<>();
// Constructors, Getters, Setter
}
@Entity
public class Student {
@Id
private String studentName;
// Constructors, Getters, Setter
}
Now, if we persist one Class
:
Class class1=new Class("name1");
class1.getStudents().add(new Student("student1Name"));
// then you can make a save of class1 in db
classRepository.save(class);
2. Unidirectional @OneToMany
with @JoinColumn
:
To fix the aforementioned extra join table issue, we just need to add the @JoinColumn
in the mix:
@OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
@JoinColumn(name = "class_id")
private List<Student> students = new ArrayList<>();
3. Bidirectional @OneToMany
:
The best way to map a @OneToMany association is to rely on the @ManyToOne side to propagate all entity state changes:
@Entity
public class Class {
@Id
private String className;
@OneToMany(
mappedBy = "class",
cascade = CascadeType.ALL,
orphanRemoval = true
)
private List<Student> students=new ArrayList<>();
// Constructors, Getters, Setter
public void addStudent(Student student) {
students.add(student);
student.setClass(this);
}
public void removeStudent(Student student) {
students.remove(student);
student.setClass(null);
}
}
@Entity
public class Student {
@Id
private String studentName;
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "class_id")
private Class class;
}
And to persist:
Class c1=new Class("className1");
c1.addStudent(new Student("StudentNAme1"));
c1.addStudent(new Student("StudentNAme2"));
c1.addStudent(new Student("StudentNAme3"));
classRepository.save(c1);
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