R – nHibernate Collection Count

collectionslazy-loadingnhibernate

I have the following model which I have created and mapped with nHibernate.
Using lazy loading so I don't need to get the Vehicles for the Dealer at the start.

Public class Dealer
{
public virtual string Name { get;set;}
public virtual IList<Vehicles> Vehicles { get;set;}
}

Now let's assume the Dealer has thousands of vehicles.

If I do Dealer.Vehicles.Count then NH will select and pull all the data.

What is the best way to simply get a count? Is there any way in which I can get a count with out declaring A new property dealerCount within the Dealer Class?

Also there is a feature in Hibernate which I believe will be implemented in a newer version of NH called Extra Lazy Loading. Would this solve the problem?

Best Answer

extra lazy loading would issue sql instead of populating the collection for certain operations such as Count or Contains. In fluent mappings its used as:

HasMany(x => x.CollectionProperty).ExtraLazyLoad();

or HBM

<one-to-many lazy="extra" ...

It's only usefull if you have large collections and need the special behavior.

Related Solutions

Java – How to filter a Java Collection (based on predicate)

Java 8 (2014) solves this problem using streams and lambdas in one line of code:

List<Person> beerDrinkers = persons.stream()
    .filter(p -> p.getAge() > 16).collect(Collectors.toList());

Here's a tutorial.

Use Collection#removeIf to modify the collection in place. (Notice: In this case, the predicate will remove objects who satisfy the predicate):

persons.removeIf(p -> p.getAge() <= 16);

lambdaj allows filtering collections without writing loops or inner classes:

List<Person> beerDrinkers = select(persons, having(on(Person.class).getAge(),
    greaterThan(16)));

Can you imagine something more readable?

Disclaimer: I am a contributor on lambdaj

Java – Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

Iterator.remove() is safe, you can use it like this:

List<String> list = new ArrayList<>();

// This is a clever way to create the iterator and call iterator.hasNext() like
// you would do in a while-loop. It would be the same as doing:
//     Iterator<String> iterator = list.iterator();
//     while (iterator.hasNext()) {
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
    String string = iterator.next();
    if (string.isEmpty()) {
        // Remove the current element from the iterator and the list.
        iterator.remove();
    }
}

Note that Iterator.remove() is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

Source: docs.oracle > The Collection Interface

And similarly, if you have a ListIterator and want to add items, you can use ListIterator#add, for the same reason you can use Iterator#remove — it's designed to allow it.

In your case you tried to remove from a list, but the same restriction applies if trying to put into a Map while iterating its content.

Best Answer

Related Solutions

Java – How to filter a Java Collection (based on predicate)

Java – Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

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