@Entity
@Table(name = "group_of_students")
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class AGroupOfStudents extends AModel {
}

@Entity
@Table(name = "cohort")
@PrimaryKeyJoinColumn(name = "id")
public class Cohort extends AGroupOfStudents {

    @Column(nullable = false)
    // @NaturalId <- here is the problem
    private int number;
}

@Entity
@Table(name = "centuria")
@PrimaryKeyJoinColumn(name = "id")
public class Centuria extends AGroupOfStudents {

    @Column(nullable = false)
    // @NaturalId <- here is the problem
    private int cohort;


    @Column(nullable = false)
    // @NaturalId <- here is the problem
    private char maniple;
}

AnnotationException: @NaturalId only valid on root entity (or its @MappedSuperclasses)