Projects/Nepomuk/QueryingMethods/en

Querying your Data

There isn't much point of pushing all your data into Nepomuk, if you cannot query it. This article mentions the different ways you can query Nepomuk and the advantages and disadvantages of each.

QueryServiceClient

The Nepomuk QueryServiceClient is an asynchronous method of running queries in Nepomuk. It accepts both Nepomuk Queries and direct SPARQL queries. It provides convenient signal which your application can connect to in order to receive the results.

Please note that the asynchronous nature of the QueryServiceClient is achieved by transmitting the query over dbus to the storage service, where a new thread is spawned, the query is run on that thread, and the results are returned over dbus. This is fine for casual use, but when you care about high performance, it is recommended that you avoid the QueryServiceClient.

Example -

    Nepomuk2::Query::Query query( .. build Query .. );
    Nepomuk2::Query::QueryServiceClient* client = new Nepomuk2::Query::QueryServiceClient( this );
    client->query( query );
    connect( client, SIGNAL(newEntries(QList<Nepomuk2::Query::Result>)),
             this, SLOT(newEntries(QList<Nepomuk2::Query::Result>)) );
    
    void TestObject::newEntries(const QList< Nepomuk2::Query::Result >& list)
    {
        foreach( const Nepomuk2::Query::Result &r, list ) {
            Nepomuk2::Resource res = r.resource();
            QHash<QUrl, Nepomuk2::Variant> prop = res.properties();
            QHashIterator<QUrl, Nepomuk2::Variant> it( prop );
            while( it.hasNext() ) {
                it.next();
                kDebug() << it.key() << " " << it.value();
            }
            kDebug() << "--------------------------";
        }
    }

The QueryServiceClient also offers ways of updating the results via entriesRemoved and newEntries. While these methods may seem very attractive, their implementation is not. They work by running the entire query when some data in the Nepomuk database changes.

In the future the queries will only be re-run when data related to the query changes (heuristics), but until then the entire query is re-run every time ANY data changes. Therefore it is recommended that you delete the QueryServiceClient when it has finished execution as follows -

    Nepomuk2::Query::QueryServiceClient* client = new Nepomuk2::Query::QueryServiceClient( this );
    connect( client, SIGNAL(finishedListing()), client, SLOT(deleteLater()) );

Result Iterator

The ResultIterator, which has been introduced in 4.10, can take either a Nepomuk or SPARQL query and provides an iterator which you can use to get the results. The iterator is obviously blocking, so you may want to put it another thread, depending on the kind of query.

Example -

    Nepomuk2::Query::Query query( .. build Query .. );
    Nepomuk::Query::ResultIterator it( query );
    while( it.next() ) {
            Nepomuk2::Resource res = it.current().resource();
            QHash<QUrl, Nepomuk2::Variant> prop = res.properties();
            QHashIterator<QUrl, Nepomuk2::Variant> it( prop );

            while( it.hasNext() ) {
                it.next();
                kDebug() << it.key() << " " << it.value();
            }
            kDebug() << "--------------------------";
        }
    }

The ResultIterator is just a light wrapper over the Soprano::QueryResultIterator. It converts the bindings returned by the QueryResultIterator into a Query::Result.

Soprano::Model

The underlying Soprano model can also be used to directly run the queries. You can only run SPARQL queries on the model. Use this method when you're writing a custom query. The underlying Model also provides an iterator similar to that of the ResultIterator.

Example -

    QString query = QString::fromLatin1("select ?r where { ?r a nco:Contact . }");
    Soprano::Model* model = Nepomuk2::ResourceManager::instance()->mainModel();
    Soprano::QueryResultIterator it = model->executeQuery( query, Soprano::Query::QueryLanguageSparql );

    while( it.next() ) {
        kDebug() << it["r"].uri();
    }

You might want to check out the Soprano API for more information about the lower level details.