@Entity
@Table(name = "base_object")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn
public abstract class BaseObject 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;
}

@Entity
public class SmallObject extends BaseObject {
    @Column(name = "second_name", nullable = false)
    private String secondName;
}

@Entity
public class BigObject extends BaseObject {
    @Column(name = "second_name", nullable = false)
    private String secondName;
    @ManyToMany
    @JoinTable(
            name = "small_object_assignment",
            joinColumns = @JoinColumn(name = "ukey1", referencedColumnName = "ukey"),
            inverseJoinColumns = @JoinColumn(name = "ukey2", referencedColumnName = "ukey")
    )
    private List<SmallObject> objects;
}