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();
}
}
}
It is easier at the moment to write data in Turtle (or N-Triples) rather than through the Model API.
model.createReifiedStatement is older functionality for reificiation which is not RDF*.
The implementation of RDF*/SPARQL* in Jena will track the work in the RDF-star community group and will need to change.
The current SPARQL* is rooted in the original RDF* paper. It uses the triple index for <<>> so matches happen using the index (it is "PG-mode" like):
<<:s :p :o>> :q :z .
Try the following data (D.ttl) (the development version of Jena has the new annotation syntax as well:
PREFIX ex: <http://example/>
ex:s ex:p ex:o .
<<ex:s ex:p ex:o>> ex:q ex:z .
and query (Q.rq):
PREFIX ex: <http://example/>
SELECT * {
<< ?s ?p ?o >> ?q ?z .
}
which, using the command line tools
sparql --data D.ttl --query Q.rq
gives
------------------------------------
| s | p | o | q | z |
====================================
| ex:s | ex:p | ex:o | ex:q | ex:z |
------------------------------------