Child object binding is null

I have this Person entity and it has a child field which is the Parent Person and join to itself via parent_id. When I tried to hit Update, why is the person.getParentPerson() is always null?

@javax.persistence.Table(name = "person")
@Entity
public class Person implements Serializable {

    // Fields
    @Column(name = "id", nullable = false, insertable = true, updatable = true, length = 20, precision = 0, unique =
        true)
    @Id
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "parent_id")
    private ParentPerson parentPerson;

    @Column(name = "name", nullable = true, insertable = true, updatable = true, length = 10)
    private String name;

    //some other fields, setters and getters
    .... 
}

@Controller
@RequestMapping("/person")
public class PersonController {

    @RequestMapping("/add.htm")
    private ModelAndView personEdit() {
        Person person = new Person();
        Person parentPerson = new ParentPerson();
        if (checkIsParent()) {
           parentPerson.setId(-1);
        }
        person.setParentPerson(parentPerson);         
        mav.addObject("person", person)
        return mav;       
    }

    @RequestMapping("/update.htm")
    @ModelAttribute("person") @Valid Person person
    private ModelAndView personUpdate() {
        //person.getParentPerson() is null   
    }
}

HTML snippet:

   <form id="uForm" action="update.htm" method="post" accept-charset="UTF-8" autocomplete="off">
       <@global.spring.formInput path="person.id" attributes='size="40"'/>
       <@global.spring.formInput path="person.name" attributes='size="40"'/>
   </form>