Re: Modeling question

[Rob Styles]

On 19 Jun 2009, at 07:34, Rob Styles wrote:

Hi Jon,

On 18 Jun 2009, at 21:10, Jon Phipps wrote:

Folks,

I'm looking for some community advice on how to best model this  
situation:

The RDA spec says that the rda:extent property applies to both
frbr:manifestation and frbr:item. Without getting into a discussion  
of
whether that's correct, I'm interested in the best way to model that.
It looks like there are at least a couple of options...

1) declare a single property with two domains:

<rda:extent> a rdf:Property ;
  rdfs:isDefinedBy <http://RDVocab.info/Elements> .
  rdfs:domain <frbr:Item> ,
                    <frbr:Manifestation> .

It's worth remembering that the rdfs:domain predicate isn't an  
imlication, not a restriction - that is it means something on the  
right of the property _becomes_ the class specified by the domain of  
the predicate, not that it is _restricted_ to resources of that class.

sorry, that should read

It's worth remembering that the rdfs:domain predicate is an  
implication, not a restriction


That means, in this case, that the object of an rda:extent property  
would become both a frbr:Item and a frbr:manifestation. Knowing the  
frbr model, I don't think that's your intention. To fix the problem  
and say that the thing one the right is either one or the other  
looks roughly like this:

<rda:extent> a rdf:Property ;
 rdfs:domain <rda:ItemOrManifestation> .

<rda:ItemOrManifestation> a owl:Class ;
   rdfs:domain [
       a owl:Class ;
       owl:unionOf [
           rdf:first frbr:Item ;
           rdf:rest [
               rdf:first frbr:Manifestation ;
               rdf:rest rdf:nil
           ]
       ]
   ] ;

This uses a bit of owl to specify a class that is the union of the  
two other classes, meaning it can be either.


2) declare a single property with two subproperties:

<rda:extent> a rdf:Property ;
  rdfs:isDefinedBy <http://RDVocab.info/Elements> .

<rda:extentItem> a rdf:Property ;
  rdfs:subPropertyOf <rda:extent> ;
  rdfs:domain <frbr:Item> .

<rda:extentmanifestation> a rdf:Property ;
  rdfs:subPropertyOf <rda:extent> ;
  rdfs:domain <frbr:Manifestation> .

I'd tend to er against this as it introduces predicates that mean  
the same thing but use different terms depending on the type of  
object, thus duplicating the type information and making it more  
complex to navigate the data without inferencing.

I think I prefer option 2 since it allows the base property to be
domain-free but still expresses the intent of the text and it's been
modeled this way elsewhere [1][2], but I'm wondering if anyone has a
good reason to prefer option 1? Or are there other ways of expressing
this that I haven't considered?

[1] when corrected, is the better way as it says what you mean  
without introducing additional predicates or duplicating the type  
information. If you;re happy to leave the extent property domain- 
free, why not just do that? That way it is far easier for people to  
re-use, and as I say the domain implies the type of the resource  
rather than restricting it.


Cheers,
Jon Phipps

[1] http://RDVocab.info/Elements/identifierForTheItem
[2] http://RDVocab.info/Elements/identifierForTheManifestation

Rob Styles
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Rob Styles
tel: +44 (0)870 400 5000
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shared innovationTM

Any views or personal opinions expressed within this email may not be those of 
Talis Information Ltd or its employees. The content of this email message and 
any files that may be attached are confidential, and for the usage of the 
intended recipient only. If you are not the intended recipient, then please 
return this message to the sender and delete it. Any use of this e-mail by an 
unauthorised recipient is prohibited.

Talis Information Ltd is a member of the Talis Group of companies and is 
registered in England No 3638278 with its registered office at Knights Court, 
Solihull Parkway, Birmingham Business Park, B37 7YB.