A unique constraint requires values in a column to occur at most once. In an object-relational mapping, it turns a foreign-key association into a database-enforced one-to-one relationship rather than relying only on application code. section 9.1.3 section 9.2.3
For a User–Address association, placing unique=true on @JoinColumn creates a SHIPPINGADDRESS_ID column with a unique constraint. No two users can reference the same address, while Hibernate can load the address lazily because the foreign-key value is available on the USERS row. section 9.1.3
One-to-one foreign-key mapping
@OneToOne(fetch = FetchType.LAZY, optional = false, cascade = CascadeType.PERSIST) @JoinColumn(unique = true) private Address shippingAddress; User john = new User("John Smith"); Address address = new Address("Flowers Street", "01246", "Boston"); john.setShippingAddress(address); userRepository.save(john); The mapping uses an ordinary foreign-key column instead of a shared primary key or special identifier generator. Saving the user transitively saves the linked address.
Apply the constraint with these considerations:
- Use a unique foreign-key column when each source entity must reference a different target entity. section 9.1.3
- Treat optionality carefully: nullable unique columns can allow multiple missing references, so they are typically not meaningful for enforcing a strict one-to-one relationship. section 9.1.3
- Use a join-table unique constraint when an optional association should avoid nullable columns; constrain the appropriate foreign-key column to encode multiplicity. section 9.2.3
