Hi Asger
Thanks for the clarification on these. I thought there would be issues with
Fedora's notion of "type" - so just wanted to clarify. And I hadn't spotted
that owl:hasValue wasn't in OWL Lite. Thanks for the suggestion on the XSD
alternative, that might just work!
Regards
Steve
-----Original Message-----
From: Asger Askov Blekinge [mailto:a...@statsbiblioteket.dk]
Sent: 13 December 2011 14:21
To: fedora-commons-users@lists.sourceforge.net
Subject: Re: [fcrepo-user] ECM - validating the "type" of an object
On 12/13/2011 01:51 PM, Stephen Bayliss wrote:
In ECM, the type of an object is defined effectively from its CModel. One
can validate the type of the target of a relationship, the typing being
specified by the hasModel relationship.
Is it possible to do any additional type validation; eg if I want to enforce
that all objects "belonging" to the CModel must assert an rdf:type
relationship?
Sure, specify the minCardinality for the rdf:type relation. That way, all
subscribing objects must have an rdf:type relationship.
I've tried adding an owl:Restriction on the rdf:type relationship using
owl:hasValue to specify the value of the target (rather than its class) -
but this doesn't look as if it is implemented.
http://www.w3.org/TR/owl-ref/#hasValue-def
NOTE: the value constraint owl:hasValue is not included in OWL Lite.
I've also tried using an owl:Restriction on a hasMember relationship;
instead of specifying the target's CModel class I used a value that the
target is itself asserting using rdf:type, but it doesn't lok like this is
tested for.
rdf:type is a sore point for OWL. Fedora CMA does not concern itself with
rdf:type. A fedora object is NOT "of a type" just because it subscribe to a
content model.
Now, one could define
fedora:hasModel isSubtypeOf rdf:type
The problem here is that, to owl, fedora:hasModel is an owl:ObjectProperty
and rdf:type is a rdf:Property. To make the above statement understandable
to a reasoner, you would need owl full.
For the purpose of ECM, i defined that
object fedora:hasModel contentmodel creates a phony rdf property
object rdf:type contentmodel#class
This property is, of course, not added to the triple store.
Remember that it does require owl full, if a content model should be both an
object and a class. That is why we use contentmodel#class for the rdf:type
information.
So a question, is ECM relationships validation restricted to:
1) checking the cardinality of a relationship
2) checking the type of the target of a relationship where the type is
specified using hasModel
Is there anything one can do for enforcing rdf:type assertions?
writing a schema for the RELS-EXT datastream?
<xsd:schema
attributeFormDefault="qualified"
elementFormDefault="qualified"
targetNamespace="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
<http://www.w3.org/1999/02/22-rdf-syntax-ns#>
xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
<http://www.w3.org/1999/02/22-rdf-syntax-ns#>
xmlns:xsd="http://www.w3.org/2001/XMLSchema";
<http://www.w3.org/2001/XMLSchema> >
<xsd:element name="RDF">
<xsd:complexType>
<xsd:sequence>
<xsd:element minOccurs="1" maxOccurs="1" name="Description">
<xsd:complexType mixed="true">
<xsd:sequence minOccurs="0" maxOccurs="unbounded">
<xsd:any processContents="lax"/>
<!-- Insert required element here...-->
</xsd:sequence>
<xsd:attribute name="about" type="xsd:anyURI"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>
But on a more serious note, no, there isn't. To do this right, we need
1. To define the implied relation between fedora:hasModel and rdf:type
2a. Find an apache-licensed reasoner, that support more than owl-lite (or
switch fedora to gpl3, and use pellet)
2b/ Or: choose to support some subset of owl full/DL specification, and thus
in effect, make our own dialect of owl.
Regards
(Though I realise in practice, with reasoning support, one would make a
statement about the CModel#class that it is eg a subtype, or owl:sameAs
etc).
Thanks
Seve
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