WebLog Pro Olivier Berger

Here’s some documentation about a hack I’ve been working on to allow publication of RDF views for Tastypie resources (in Django applications).

While working on implementing support for RDF meta-data for descriptions of Debian packages for the Debian Package Tracking System rewrite, I’ve tried and idetify which libraries/frameworks would allow to create some RDF views for Django model objects with a minimal effort.

Ideally, this would save the hassle of writing code, and could just be a matter of mapping some Django model fields to proper ontology attributes.

I haven’t found an existing tool to do so, but it seemed to me that Tastypie could offer a nice starting point. Tastypie (I focused on v. 0.9.15 which is currently in Debian testing) offers some REST content-negociation support, and other niceties.

This post is an attempt at documenting an initial implementation for my problem. I’ve implemented it as some code in the example blog application described in the Tastypie documentation. Unfortunately, it’s not yet a patch that could be applied to Tastypie, to add this as a standard feature.

The code is available in my git clone of Tastypie, based on 0.9.15, and lies in the commit(s) between the example_myapp and rdfviews branches. It needs an up to date RDFLib to work (Debian’s is too old, btw).

It basically relies on the addition of a _rdf_mapping dict in the Django model objects, and an RdfModelResource as a base class for Tastypie ModelResources.

To provide an RDF view for a ModelResource, it also requires to declare a particular MetaClass, and adding a few bits to its Meta subclass.

The principle is that the dehydratation steps of the Tastypie system will replace fields values by their RDF objects counterparts, as either RDFLib Literals or URIRefs. Then the resuting Bundle will be converted to some RDFLib Graph. It is then just a matter of serialization of that Graph to turtle (I’ve not added another format, but it should be pretty straightforward).

There’s a bit of hackish drak magic in the code, in particular when introspecting the Tastypie / Django resource / model objects to be able to dynamically add some dehydrate_FOO methods to perform the conversion to Literals or URIRefs.

Custom dehydrate_Foo methods can still be written to process the fields in particular ways.

I’ve added some LDP style paging along the way, so I hope it will be quite usable by LDP compatible tools.

Here’s the result :

First, listing all resources :

$ curl -H 'Accept: text/turtle' "http://localhost:8000/api/v1/entry/"
@prefix dcterms: <http://purl.org/dc/terms/> .
@prefix ldp: <http://www.w3.org/ns/ldp#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix sioc: <http://rdfs.org/sioc/ns#> .
@prefix xml: <http://www.w3.org/XML/1998/namespace> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

</api/v1/entry/#list> dcterms:hasPart </api/v1/entry/1/#post>,
</api/v1/entry/2/#post>,
</api/v1/entry/3/#post> .

Then, details of one particular post :

$ curl -L -H 'Accept: text/turtle' "http://localhost:8000/api/v1/entry/1"
@prefix dcterms: <http://purl.org/dc/terms/> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix sioc: <http://rdfs.org/sioc/ns#> .
@prefix xml: <http://www.w3.org/XML/1998/namespace> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

</api/v1/entry/1/#post> a sioc:Post ;
dcterms:created "2011-05-21T22:46:38+00:00"^^xsd:dateTime ;
dcterms:title "Another Post" ;
sioc:content "MESSAGE: This will prbbly be my lst post. /MESSAGE" ;
sioc:has_creator </api/v1/user/1/> ;
owl:sameAs </posts/another-post#post> .

I hope this is usable, and welcome any feedback.