Can I use a SPARQL* query to query a classic RDF model that uses reification? To me, the Jena documentation is a bit vague here.
The code below creates a reified statement:
<< <http://www.mysubject.com> <http://www.mypredicate.com> <http://www.myobject.com> >> <http://www.sayed.de#sayed> <http://www.sayer.de> .
The code further contains two queries: (i) A classic SPARQL query, (ii) a SPARQL* query.
Both query for <http://www.sayer.de> as result. While (i) comes back with the solution, (ii) fails to do so.
What am I doing/understanding wrong here?
import org.apache.jena.ontology.DatatypeProperty;
import org.apache.jena.ontology.OntModel;
import org.apache.jena.ontology.OntModelSpec;
import org.apache.jena.query.*;
import org.apache.jena.rdf.model.*;
import java.io.StringWriter;
public class RdfStar {
public static void main(String[] args) {
// let's create an ontModel and fill it with data:
OntModel model = ModelFactory.createOntologyModel(OntModelSpec.OWL_MEM);
ReifiedStatement s = model.createReifiedStatement(
model.createStatement(
ResourceFactory.createResource("http://www.mysubject.com"),
ResourceFactory.createProperty("http://www.mypredicate.com"),
ResourceFactory.createResource("http://www.myobject.com"))
);
ObjectProperty sayedProperty = model.createObjectProperty("http://www.sayed.de#sayed");
s.addProperty(sayedProperty, model.createResource("http://www.sayer.de"));
// write to console
StringWriter myWriter = new StringWriter();
model.write(myWriter, "NT");
String result = myWriter.toString();
System.out.println(result);
// now let's create a regular query
String queryString = "SELECT ?who WHERE {" +
"?statement <http://www.sayed.de#sayed> ?who ." +
"}";
Query query = QueryFactory.create(queryString) ;
try (QueryExecution qexec = QueryExecutionFactory.create(query, model)) {
ResultSet results = qexec.execSelect();
while (results.hasNext()) {
QuerySolution soln = results.nextSolution();
RDFNode x = soln.get("who");
System.out.println(x);
}
} catch (Exception e){
e.printStackTrace();
}
// NOW SPARQL STAR
queryString = "SELECT ?who WHERE {" +
"<< ?a ?b ?c >> ?d ?who ." +
"}";
query = QueryFactory.create(queryString, Syntax.syntaxARQ) ;
try (QueryExecution qexec = QueryExecutionFactory.create(query, model)) {
ResultSet results = qexec.execSelect();
while (results.hasNext()) {
QuerySolution soln = results.nextSolution();
RDFNode x = soln.get("who");
System.out.println("Star result: " + x);
}
} catch (Exception e){
e.printStackTrace();
}
}
}
