Why does updating children of a record, remove the connection with its children?

Question

I need to update the items of the following class, although I can update all the items but all fields of the class will be changed to null, and its connection to its members will be removed.

@Entity
public class Category implements Serializable {
    @Id
    @GeneratedValue
    private long id;

    @OneToMany( cascade = CascadeType.ALL, orphanRemoval = true)
    @LazyCollection(LazyCollectionOption.FALSE)
    private List<Items> items;

    private float price;

     @ManyToOne
    private Staff user;

     @ManyToOne
    private Staff owner;

    @Temporal(javax.persistence.TemporalType.DATE)
    private Date createDate;
    ....

@Entity
public class Items {

    @Id 
    @GeneratedValue
    private Long id;

    private int serial;

    @OneToOne
    private Product product;
    .....

Hibernate

            Category cat = (Category) session.get(Category.class, id);
            for (int i = 0; i < cat.getItems().size(); i++) {
                    cat.getItems().get(i).getProduct().setQuantity(10);

                    Part part = new Part();
                    part.setName("Temp");
                    cat.getItems().get(i).getProduct().getPart.add(part);

                    session.update(cat.getItems().get(i).getProduct());
                    session.save(part);

            }
            tx.commit();

Example

Before Update

Category 
1 2000 Alex Jack 05-05-2014

category_categoryitem

1 137

categoryitem

137 900 20

After Update

category
1 0 Null Null Null

category_categoryitem


categoryitem 

137 900 20

Answer 1

The code is correct, except that I just added @DynamicUpdate to avoid updating unmodified fields, and removed a line which was updating the table from another function.

Answer 2

Have you tried using @JoinTable to link Item and Category?

Category.java

...
@OneoMany(cascade=CascadeType.ALL, fetch = FetchType.LAZY)
@JoinTable(
  name="category_items",
  joinColumns={@JoinColumn(name="category", referencedColumnName="id")},
  inverseJoinColumns={@JoinColumn(name="item", referencedColumnName="id")})
private Set<Items> items = new HashSet<Items>();
...