@Embeddable
public class AuthorId implements Serializable {

	private String id;

	public AuthorId(String id) {
		this.id = id;
	}
        
         ....
}

@Embeddable
public class AuthorId implements Serializable {

        @Column(name = "_id")
	private String id;

	public AuthorId(String id) {
		this.id = id;
	}
        
         ....
}