Hibernate ManyToOne vs OneToOne

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.

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.