@Entity
@Table(name = "animal")
public class Animal implements Serializable {

    @Id
    @Column(name = "id")
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;
    @Column(name = "name", nullable = false, unique = true)
    private String name;

    @Column(name = "ukey", unique = true)
    private String key;
    
    @ManyToMany
    @JoinTable(
            name = "relatives",
            joinColumns = @JoinColumn(name = "ukey1", referencedColumnName = "ukey"),
            inverseJoinColumns = @JoinColumn(name = "ukey2", referencedColumnName = "ukey")
    )
    private Set<Animal> relatives;
}