@Entity
@Table(name = "client")
@Data
public class Client {
....
@OneToOne(mappedBy = "client", fetch = FetchType.LAZY, optional = false)
private Passport passport;

@Entity
@Table(name = "passport")
@Data
public class Passport {
...
@JsonIgnore
@OneToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "client_id", nullable = false, unique = true)
private Client client;

Client client = clientRepository.findOne(id);

Hibernate: 
    select
        client0_.id as id1_0_0_,
        client0_.first_name as first_na2_0_0_,
        client0_.last_name as last_nam3_0_0_ 
    from
        client client0_ 
    where
        client0_.id=?
Hibernate: 
    select
        passport0_.id as id1_2_0_,
        passport0_.client_id as client_i3_2_0_,
        passport0_.number as number2_2_0_ 
    from
        passport passport0_ 
    where
        passport0_.client_id=?

Passport passport = passportRepository.findOne(id);

        Client client = passport.getClient();
        logger.info("client id = " + client.getId());

public class Client
@OneToOne(mappedBy = "client", fetch = FetchType.LAZY, optional = false, cascade = CascadeType.ALL)
private Passport passport;

@OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "client_id", unique = true)
    private Client client;