Failing to insert the foreign key in the owning table when save method is being fired from the non-owning object in a bidirectional relationship?

Question

So I have Two entities 'Student' and 'Parents' and the relationship between them is of 'OneToOne'.

Owning side here is 'student'.

Student.java

import java.util.UUID;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToOne;
import javax.persistence.Table;

import org.hibernate.annotations.Type;

@Entity
@Table(name="student")
public class Student {
    
    @Id
    @Column(name="id")
    @GeneratedValue(generator="pg-uuid",strategy=GenerationType.AUTO)
    @Type(type = "org.hibernate.type.PostgresUUIDType")
    private UUID id;
    
    @Column(name="name")
    private String name;
    
    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name="parent_id")
    private Parents parents;

        Student(){
    }
    
    Student(String name){
        this.name=name;
    }

    //Getters And Setters

Parents.java

import java.util.UUID;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.Table;

import org.hibernate.annotations.Type;

@Entity
@Table(name="parents")
public class Parents {
    
    @Id
    @Column(name="id")
    @GeneratedValue(generator="pg-uuid",strategy=GenerationType.AUTO)
    @Type(type = "org.hibernate.type.PostgresUUIDType")
    private UUID id;
    
    @Column(name="father_name")
    private String fatherName;
    
    @Column(name="mother_name")
    private String motherName;
    
    @OneToOne(mappedBy = "parents",cascade = CascadeType.ALL)
    private Student student;

        public Parents() {
    }

    public Parents(String fatherName,String motherName) {
        this.fatherName=fatherName;
        this.motherName = motherName;
    }
    //Getter and Setters

Database : POSTGRESQL

Student table

  Column   |         Type          | Collation | Nullable |      Default
-----------+-----------------------+-----------+----------+--------------------
 id        | uuid                  |           | not null | uuid_generate_v4()
 name      | character varying(50) |           |          |
 parent_id | uuid                  |           |          |
Indexes:
    "student_pkey" PRIMARY KEY, btree (id)
    "student_parent_id_key" UNIQUE CONSTRAINT, btree (parent_id)
Foreign-key constraints:
    "student_parent_id_fkey" FOREIGN KEY (parent_id) REFERENCES parents(id)

Parents table

   Column    |         Type          | Collation | Nullable |      Default
-------------+-----------------------+-----------+----------+--------------------
 id          | uuid                  |           | not null | uuid_generate_v4()
 father_name | character varying(80) |           | not null |
 mother_name | character varying(80) |           | not null |
Indexes:
    "parents_pkey" PRIMARY KEY, btree (id)
Referenced by:
    TABLE "student" CONSTRAINT "student_parent_id_fkey" FOREIGN KEY (parent_id) REFERENCES parents(id)

Note:-

1.) I'm using uuid_generate_v4() as default in both the tables for auto generating UUID

2.)parent_id in the student table is the foreign key that has reference for parents table primary key;

3.) Both the entities are in a bidirectional relationship.

My StudentApp.java

public class StudentApp {
    public static void main(String args[])
    {
        SessionFactory factory=new Configuration()
                .configure()
                .addAnnotatedClass(Student.class)
                .addAnnotatedClass(Parents.class)
                .buildSessionFactory();
        
        Session session=factory.getCurrentSession();
        
        try {
            
            Student student=new Student("Sinchan Nohara");
            Parents parents=new Parents("Hiroshi Nohara","Missy Nohara");
            
                        student.setParents(parents);
 
            session.beginTransaction();
            
            session.save(student)
            
            session.getTransaction().commit();
        }
        catch(Exception e){
            e.printStackTrace();
        }
        finally{
            session.close();
            factory.close();
        }
                
    }
}

So if I run the code given below in my StudentApp.java

Student student=new Student("Sinchan Nohara");
Parents parents=new Parents("Hiroshi Nohara","Missy Nohara");
            
student.setParents(parents);
 
session.beginTransaction();
            
session.save(student)
            
session.getTransaction().commit();

its working fine .

Student table (Owning side)

                  id                  |      name      |              parent_id

--------------------------------------+----------------+--------------------------------------
89879cd1-a562-48f3-8632-9eaa5d288f5f | Sinchan Nohara | 82206833-b0d0-4e99-94f5-cab32he8a361

Parents Table (Referenced Side)

                  id                  |  father_name   | mother_name
--------------------------------------+----------------+--------------
 82206833-b0d0-4e99-94f5-cab32he8a361 | Hiroshi Nohara | Missy Nohara

But when I try to run the code given below in my StudentApp.java

Student student=new Student("Sinchan Nohara");
Parents parents=new Parents("Hiroshi Nohara","Missy Nohara");
            
parents.setStudent(student);
 
session.beginTransaction();
            
session.save(parents)
            
session.getTransaction().commit();

then the foreign key(parent_id) in the student table is not getting inserted.

look at the tables below now,

Parents table

                  id                  |  father_name   | mother_name
--------------------------------------+----------------+--------------
 380e4daa-0cba-4e26-aa3f-446dbc4fd2c0 | Hiroshi Nohara | Missy Nohara

Student Table

                  id                  |      name      | parent_id
--------------------------------------+----------------+-----------
 de062q8d-ff58-4c05-a342-7uu67a4c842d | Sinchan Nohara |

Why this is happening? and how to fix it?

Answer 1

Dealing with bidirectionnal relations with hibernate is tricky. In fact, your model shows that the relation is owned by the student. When you define the relation, it is not enough to set the student of a parents to make it work. You have to set the parents of the student. This is tricky because when you load a student or a parents you will get both objects with their bidirectional relation properly set. The problems arise when you do create, update or delete. That's where the owner of the relation becomes important. By the way, in your case, I would rather set the parents as the owner of the relation to the student since, I imagine, parents may exist without student and not the opposite.