Hibernate, insert or update without select

Question

I have a products objects which belongs to certain categories i.e. classical many to one relationship.

@Entity
public class Product{

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    Long id;

    String name;
    Double price;
    @ManyToOne(fetch = FetchType.LAZY)
    Category category;
...
}

@Entity
public class Category implements Identifiable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String name;
...
}

I want to insert and update products without preselecting categories. Like this:

Product product = dao.get(productId);
Category category = dao.get(categoryId);
product.setCategory(category);
dao.update(product);

or

Product product = new Product(somename);
Category category = dao.get(categoryId);
product.setCategory(category);
dao.insert(product);

Is it possible to update and insert without category selecting? I don't want to use HQL or direct queries for that.

Answer 1

session.load() exists specifically for cases like this. The following:

Category category = session.load(categoryId);
product.setCategory(category);

will not hit the database. It will, however, throw an exception at a later stage (during flush, more or less) if there is no category available with given id.

Using load() is faster than merge() and has no side-effects (cascading, etc...)

Answer 2

Hibernate needs a safe way to determine the state of your object. That's why it's going a long way to make sure you can't mix objects from different sessions (so you can't load the categories at startup and then just use them later).

The solution is to load the categories at startup, set up a caching provider like ehcache for this class and then use this code:

Product product = new Product(somename);
product.setCategory(session.merge(category));

That won't cause any DB round trips if you configure the cache that category instances can't change.