Add a Vocabulary for DC Elements

Build an RDF Graph

We’ll switch gears here and see how we can build up an RDF graph using RDF.ex rather than writing out the RDF explicitly. We’ll aim to generate the book graph using the semantic RDF form we already discussed.

URI strings

One of the challenges to working with RDF is managing URI strings. We need to deal with node names ourselves for instance data because these generally use an application-specific naming. But for edges and node labels, we don’t want to be dealing with URI strings every time we add a property or a class name.

We can make use of one particular aspect of RDF.ex—its support for RDF vocabularies. The distribution ships with five vocabularies (RDF, RDFS, OWL, SKOS, and XSD) already included. These vocabularies, however, provide terms from RDF schema languages for general data structuring rather than for capturing any specific semantics. But we can easily add in new vocabularies with their own semantics.

DC elements vocabulary

Let’s look first at the core DC elements vocabulary because this is very simple: 15 properties only. In fact, with this small number, we can easily list these out by hand. We’ll create a module called RDFGraph.Vocab in the lib/rdf_graph/vocab.ex location. We add a use RDF.Vocabulary.Namespace declaration and also a defvocab definition block for DC: