How to add a product in a list without loading all the database ?

Question

Hi,

In my domain model I have a bi-directionnel association between the ProductList entity and the Product entity with the following hibernate mapping :

@Entity @Indexed
@Table(name="product_list")
public class ProductList {

@ManyToMany(fetch=FetchType.LAZY)
@JoinTable(name = "list_items",
        inverseJoinColumns = { @JoinColumn(name = "product_id")},
        joinColumns = { @JoinColumn(name = "list_id")})
@IndexColumn(name = "item_index", base = 1, nullable = false )
@LazyCollection(LazyCollectionOption.EXTRA)
@BatchSize(size=50)
private List<Product> products = new LinkedList<Product>();
....

}

@Entity
@Table(name="logical_item")
@Cache(usage=CacheConcurrencyStrategy.READ_WRITE)
public class Product {

@ManyToMany(fetch=FetchType.LAZY, mappedBy="products")
private Set<ProductList> productLists = new LinkedHashSet<ProductList>();

...
}

But when I tried to add a product to a persistent productList Hibernate try to load all the product in the list before ! I have more than 14 000 products in a list !

Product item = (Product) session.get(Product.class, 123);
ProductList myFavoriteItems = (ProductList) session.get(ProductList.class, 321);

// Evil lazy loading (need more 512Mo of memory )
myFavoriteItems.addItem(Product item);

public void addItem(Product item){
    this.getProducts().add(item);
    item.getProductLists().add(this);
}

How to add a product in a list without loading all the database ?

Answer 1

i guess that it is one of the drawbacks of using a ManyToMany relation when you need to update the join table.

I would advise creating an entity out of the join table, and then you would only had to create the join entity and save it :

public class ProductListItem {
    @ManyToOne(...)
    private Product product;

    @ManyToOne(...)
    private ProductList productList;

    ...
}

And you could still have a transient getter than would return a product list from a product :

public class Product {

    @OneToMany(...)
    private Set<ProductListItem> items;

    @Transient
    public Set<ProductList> getProductLists() {
        Set<ProductList> list = new LinkedHashSet<ProductList>();
        for(ProductListItem item : items) {
            list.add(item.getProductList());
        }
        return Collections.unmodifiableSet(list);
    }
    ...
}

Same thing for the other side of the manytomany relation.

Then, your save operation is just a matter of creating a ProductListItem and saving it, which will load nothing, and need only one insert.

Be careful with your already existing hql queries : if they used the link Product<->ProductList, they won't work anymore.

if you wish to keep the ManyToMany relation, you should look at : http://josephmarques.wordpress.com/2010/02/22/many-to-many-revisited/ (i've never tried this solution)

Answer 2

public class Controller {

private static SessionFactory sf = HibernateUtil.getSessionFactory();

/**
* @param args
*/
public static void main(String[] args) {
// construct data
sf.getCurrentSession().beginTransaction();
Item i1 = new Item("i1");
Item i2 = new Item("i2");
Item i3 = new Item("i3");
Category c1 = new Category("c1");
sf.getCurrentSession().save(i1);
sf.getCurrentSession().save(i2);
sf.getCurrentSession().save(i3);
sf.getCurrentSession().save(c1);
c1.getItems().add(i1);
i1.getCategories().add(c1);
c1.getItems().add(i2);
i2.getCategories().add(c1);
sf.getCurrentSession().getTransaction().commit();

// get Category & i (i3)
sf.getCurrentSession().beginTransaction();
Category c = (Category) sf.getCurrentSession().get(Category.class, c1.getId());
Item i = (Item) sf.getCurrentSession().get(Item.class, i3.getId());

// proxys i & c have null Set
System.out.println("i : " + i.getName());
System.out.println("c : " + c.getName());

// here we have the IDs
long category_id = c.getId();
long item_id = i.getId();

sf.getCurrentSession().getTransaction().commit();

// add many to many data
sf.getCurrentSession().beginTransaction();

// here we can use pure SQL to add a line in CATEGORY_ITEM.
// with the known IDs
String ins = "insert into category_items (item_id,category_id,item_index) SELECT 4639, 100, MAX(item_index)+1 from category_items where category_id = 100 ;";

sf.getCurrentSession().getTransaction().commit();

}

}