@Entity
public class Salon {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "custom_sequence")
    @SequenceGenerator(name = "custom_sequence", sequenceName = "custom_sequence", allocationSize = 1)
    @Column(nullable = false)
    private Long id;

    @OneToOne(mappedBy = "salon", cascade = CascadeType.ALL, orphanRemoval = true)
    private Subscription subscription;
}

@Entity
public class Subscription {

    @Id
    private Long id;

    @OneToOne
    @MapsId
    private Salon salon;
}

    @Test
    public void test() {
        Salon salon = generateRandomSalon();
        Subscription subscription = new Subscription();
        salon.setSubscription(subscription);
        salonService.save(salon);
    }