Add support to create RDF* statements

[pajoma]

Is your feature request related to a problem? Please describe.
I am using RDF4J as backend with RDF* statements in the form

<triple> predicate value.

I am unable to convert these statements in RDFNQuads, since the subject is expected to be a RdfResource

Describe the solution you'd like
The RdfTriple interface could extend the RdfResource (example from RDF4J), to create RDFNQuads with RDF* semantics.

Describe alternatives you've considered
Not sure, for now I am filtering all RDF* statements.

[filip26]

@pajoma I'm sorry, but this project has nothing to do with RDF4J. Report the bug to RDF4J. Please feel free to re-open if there is something I'm missing.

[pajoma]

Sorry for the confusion, I just mentioned RDF4J as example.

The problem lies within the RDF Api of Titanium (RDFNQuads and Rdf Resource are Titanium Terms, nothing related to RDF4J)

[filip26]

Please, can you be more specific then? Why you cannot use the existing RDF API? RdfNQuad just extends RdfTriple of graph name which can be null.

[pajoma]

Sure.

RDF statements are triples in the form <subject> <predicate> <object>.
RDF quads include the context: <subject> <predicate> <object> <graph>.

RDF* allows for expressing statements about statements (in subjects or objects):

<<subject> <predicate> <object>> <predicate> <object> or
<subject> <predicate> <<subject> <predicate> <object>>

A valid triple (let's forget about the graph for now) is therefore:
<Resource> <IRI> <Value> with

• Resource being either a blank node, an IRI or a triple (RDF*)
• Value being either a blank node, an IRI, a triple or a literal

You typically see this reflected in the class hierarchy, see RDFLib, Eclipse RDF4J or Apache Jena

In my application, I want to expose my triples as JSON-LD documents with RDF-Star statements. The implementation follows this pattern:

for each statement in the result model:

1. convert the subject into a RdfResource
2. convert the object into a RdfValue
3. create a new RDFNQuad
4. Add it to the new RdfDataset

And then I build a JsonLd Document

Document document = RdfDocument.of(myRdfDataset);
JsonArray jsonValues = JsonLd.fromRdf(document).options(myOptions).get();
JsonDocument jsonDocument = JsonDocument.of(jsonValues);
JsonObject jsonObject = JsonLd.frame(jsonDocument, myContectUri).options(myOptions).get();

The issue:
The RdfTriple class expects a RdfResource as subject (and a RdfValue as object). Since RDF* requires triples as subjects and objects, the Triple itself should be a RdfResource itself again.

I would expect the following class structure:

• RdfValue
  • RdfLiteral
  • RdfResource
    • RdfTriple
      • RdfNQuad

Only then is it possible to create a new RDFNQuad with a triple either as subject or object (this should also include a check that it is actually a triple, and not another nquad)

[filip26]

should be done after/together with/ #300