Graphs in RDFΒΆ

In RDF, the notion of what constitutes a graph is not well defined. It is somewhat analogous to a document when thought of as “the collection of triples obtained by dereferencing the document’s URI, but not all graphs may be obtained in that way. For example, the result of a CONSTRUCT or DESCRIBE SPARQL query typically returns an unnamed graph. In N3 inferencing rules, such as:

{ ?x a Cat } => { ?x a Animal } .

both the left and right hand sides of the rules are themselves embedded or “quoted” graphs.

Some of our initial reasons for considering how to talk about graphs are discussed in the Change Sets section of this manual.

The rdfg vocabulary gives us some concepts for talking about graphs. It is very small, so quoted in its entirety below:

@prefix rdf: <> .
@prefix rdfs: <> .
@prefix rdfg: <> .

rdfg:Graph a rdfs:Class ;
    rdfs:label "Graph" ;
    rdfs:comment "An RDF graph (with intensional semantics)." .

rdfg:equivalentGraph a rdf:Property ;
    rdfs:label "equivalent graph" ;
    rdfs:comment "The graphs associated with the subject and object are
                  equivalent." ;
    rdfs:domain rdfg:Graph ;
    rdfs:range rdfg:Graph .

rdfg:subGraphOf a rdf:Property ;
    rdfs:label "subgraph of" ;
    rdfs:comment "The graph associated with the subject is a subgraph of
                  a graphequivalent to that associated with the object." ;
    rdfs:domain rdfg:Graph ;
    rdfs:range rdfg:Graph .

This is useful as far as it goes, but there seem to be two problems with it. First it is a completely closed vocabulary, that is there is a class and two predicates for talking about that class but no way of referring to the class as such from outside. Second, and this is likely intentional on the part of the authors, there is no inverse of rdfg:subGraphOf.

To tackle the first problem (in the context of Change Sets) we define:

@prefix ordf: <> .

ordf:graph a rdf:Property ;
    rdfs:range rdfg:Graph ;
    rdfs:label "graph" ;
    rdfs:comment "Reference to a graph" .

The second problem is harder and what we are trying to accomplished is best explained with reference to Bibliographica a website built with openbiblio a Pylons application that uses ORDF. Without going into the intricacies of conceptual frameworks for organisation of libraries, we have two fundamental entities that we’re concerned with: People (authors, translators, publishers, etc.) and Works (books, musical scores, recordings, etc.).

We have chosen that each individual entity merits its own URI, its own Graph in the system. So each Work, each Person has a page with some statements about them.

Now users of the system are going to want to see, most often, not just biographical information about a Person but the list of works (with some metadata) that they are responsible for. Or they will want to see complete information about a work together with abbreviated information about the authors and publishers.

Because we allow editing of the data in a wikiesque way, it is very important that we keep straight where any triples being added or modified belong, and the best way to do this is to ensure that they only exist in one place.

So need a way to specify the construction of conjunctive graphs, to use the terminology from RDFLib. But we don’t want to explicitly construct them, we want the client to take care of that.

So we might have something like this:

DostoyevskyAndWorks a rdfg:Graph ;
    ordf:subGraph Dostoyevsky ;
    ordf:subGraph CrimeAndPunishment ;
    ordf:subGraph TheIdiot .

and the client would see this and look at all the ordf:subGraph triples and then pull them in by reference. Of course we have to define this predicate:

ordf:subGraph a rdf:Property ;
    rdfs:label "sub-graph" ;
    rdfs:comment "Indicates a sub-graph containing statements belonging
                  also to this rdfg:Graph" ;
    rdfs:domain rdfg:Graph ;
    rdfs:range rdfg:Graph .

Note that it is not possible (or desirable in our application) to write the inference rule, “If S is a sub-graph of G –> all statements in S are also in G”.

This also brings up an interesting way of handling which Fresnel Lenses are intended to be used to view a resource. Noticing the lack of a general predicate for referring to lenses, we define ordf:lens, and apply it in our conjunctive graph:

ordf:lens a rdf:Property ;
    rdfs:label "lens" ;
    rdfs:comment "Which Fresnel lens to use when displaying this graph" ;
    rdfs:domain rdfg:Graph ;
    rdfs:range fresnel:Lens .

DostoyevskyAndWorks ordf:lens AuthorAndWorksLens .

In the case of specialised aggregate graphs it would even be quite possible to define the lens directly therein.

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