Hibernate ManyToOne vs OneToOne

Question

I can't see any difference in the schema of a Many-To-One relationship vs a OneToOne relationship:

@Entity public class Order {      @ManyToOne     @JoinColumn(nullable = false)     private Address address; 

vs

@Entity public class Order {      @OneToOne     @JoinColumn(nullable = false)     private Address address; 

Is there any difference?

Answer 1

They look exactly the same on schema but there is difference on Hibernate Layer.

If you try something like that:

Address address = new Address(); Order order1 = new Order(); order1.setAddress(address); Order order2 = new Order(); order2.setAddress(address); save(); 

Everything will be OK. But, after save if you try get Order:

@OneToOne case: org.hibernate.HibernateException: More than one row with the given identifier was found: 1  @ManyToOne case: SUCCESS 

Of course, your Address class should looks different in both cases.

Answer 2

There should normally be a unique constraint on the address_id join column in the case of a OneToOne association, to guarantee that only one Order can have a given address.