[FIXED] what is the reason that Hibernate does not allow to save the object which references an unsaved transient instance?

Issue

I'm newbie in Hibernate and I'm trying to learn about JPA and Hibernate.

I want to know that what is the reason that Hibernate does not allow to save the object which references an unsaved transient instance? I want to know WHY this is a problem?

I asked someone and some of them answer me like this:

How could we possibly map the customer to the address, if there is no adress record in the DB yet?

and

you are assigning particular Address to Customer. But Address does not have any ID

but honestly I can't understand them.

(I know that an exception will be thrown and the solution is Cascade but I want to the reason of the problem inside the database)

now, let's assume we have all of these code:

(I use Bidirectional One-To-One relationship for my example)

public class Customer {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String firstName;
    private String lastName;


    @OneToOne(mappedBy = "customer")
    private Address address;
}


@Entity
public class Address { 

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String street;
    private String zipCode;

    @OneToOne
    private Customer customer;
}

   public static void main(String[] args) {

        EntityManager entityManager = emf.createEntityManager();
        entityManager.getTransaction().begin(); // Begin Transaction

        Customer c1 = new Customer("Mi", "S");
        Address addrss1 = new Address("5412 S 5th", "212524");

        c1.setAddress(addrss1);
        addrss1.setCustomer(c1);


        entityManager.persist(c1);

        entityManager.getTransaction().commit(); // Commit 
        entityManager.close();
    }

and let's assume that the exception is not thrown and java and hibernate have allowed us to run our code and this is our customer table.

   id    firstName    lastName    
---------------------------------
    1        Mi           S

and this is our address table:

   id    street    zipCode    customer_id
---------------------------------------------
    -      -         -            -

now, what is the problem? everything in these Bidirectional One-To-One relationship seems right. then what is the problem?

---

PS: if it is possible, please explain and show me code. I can understand better with code. thank you.

I want to see for example if we are allowed to save the object which references an unsaved transient instance, what problems will we face in our code and in our tables (for example do we have any problem when we want to retrieve a customer and etc)

Solution

Because your adress entity have the primay key of customer as a foreign key ,(since mappedby is in Customer entity) ,and the customer referenced by the adress has no id ,which tells hibernate that that entity was never persisted in the database (which literally means transient) ,and hibernate needs a persisted/managed entity to make sure it exists in the database so that the adress object can be associated with an existing customer.

Answered By - karim farhouti
Answer Checked By - Pedro (JavaFixing Volunteer)