refactoring - Hibernate - How to persist a new item in a Collection without loading the entire Collection

Question

I have a collection in my model that contains a set of 'previous versions' of my root domain object. The previous versions are therefore 'immutable' and we will never want to update them, and only want to add past versions as they arise. Also the 'versioned' domain object is fairly complex and causes heavy database access to retrieve.

When I have a new version of one of these objects I want to save it with the others without loading the entire set. The Advanced FAQ has some advice on this:

Why does Hibernate always initialize a collection when I only want to add or remove an element?

Unfortunately the collections API defines method return values that may only be computed by hitting the database. There are three exceptions to this: Hibernate can add to a <bag>, <idbag> or <list> declared with inverse="true" without initializing the collection; the return value must always be true.

If you want to avoid extra database traffic (ie. in performance critical code), refactor your model to use only many-to-one associations. This is almost always possible. Then use queries in place of collection access.

I am new to all of this and am not 100% sure on how to refactor your model to use only many-to-one associations. Can anyone please give me an example of point me to a tutorial so that I can learn how this will resolves my issue?

Answer 1

When you have a List or Set-based collection and you add a new object into your collection, Hibernate will always hit the database because it compare one by one object by using equals implementation before saving or updating - when using a Set - or by comparing a index column when using a List. This behavior is needed because of the Set and List semantic. Because of that, the performance of your application can decrease significantly whether you have a bunch of records.

Some workaround to overcome this issue

1o Conversion pattern by using a encapsuled Bag collection plus your desired Set or List exposed as a property

@Entity
public class One {

    private Collection<Many> manyCollection = new ArrayList<Many>();

    @Transient
    public Set<Many> getManyCollectionAsSet() { return new HashSet<Many>(manyCollection); }
    public void setManyCollectionAsSet(Set<Many> manySet) { manyCollection = new ArrayList<Many>(manySet); }

    /**
      * Keep in mind that, unlike Hibernate, JPA specification does not allow private visibility. You should use public or protected instead
      */
    @OneToMany(cascade=ALL)
    private Collection<Many> getManyCollection() { return manyCollection; }
    private void setManyCollection(Collection<Many> manyCollection) { this.manyCollection = manyCollection; }

}

2o Use ManyToOne instead of OneToMany

@Entity
public class One {

    /**
      * Neither cascade nor reference
      */

}

@Entity
public class Many {

    private One one;

    @ManyToOne(cascade=ALL)
    public One getOne() { return one; }
    public void setOne(One one) { this.one = one }

}

3o Caching - when applied because of, depending on your requirements, your configuration can increase or decrease the performance of your application. See here

4° SQL constraint - If you want a collection that behaves like a Set, you can use a SQL constraint, which can be applied to a column or set of columns. See here