Re: shapes-ISSUE-168 (instance count): How to constrain number of instances of a class in a graph [SHACL - Core]

Hi Dimitris,

the body of this new constraint component would be something like

SELECT $this
WHERE {
     ?instance rdf:type/rdfs:subClassOf* $this .
} HAVING (COUNT(?instance) > $maxInstanceCount)

The scope is generally binding the values of $this. So in order to make 
this work, most people would use sh:scopeNode to link to a class 
directly. Use cases for sh:scopeClass would be in the realm of 
metaclasses, or custom scopes such as "for all classes that have a given 
property" (sh:scopeProperty). (I hope this addresses your other 
questions below).

BTW if we merge Shape and NodeConstraint, as suggested, the syntax would 
look better:

ex:MyShape
     a sh:Shape ;
     sh:scopeNode schema:FlightReservation ;
     sh:maxInstanceCount 1 ;
     sh:minInstanceCount 1 .

The general problem with scope-based constraints is that scopes are only 
one way to trigger validation. Other cases include nested shapes 
(sh:shape, sh:or etc) and (as in ShEx) to have some dedicated root shape 
or (as in Arthur's use cases, if I may speak on his behalf) control the 
validation process completely outside of the model, from some 
application-specific algorithm. So anything that behaves differently 
depending on whether it has a scope or not is bound to fail.

Holger


On 14/06/2016 3:33, Dimitris Kontokostas wrote:
> Hi Holger,
>
> I think there is a typo in your example, it should be sh:scopeClass.
>
> For me it is fine to have this as syntactic sugar for Karen's use case 
> but we need to be more precise on how this will work with scoping in 
> general.
> Does this constraint apply on the union of all defined scopes or per 
> defined scope ?
>
> e.g. consider this example
> ex:MyPersonShape
>     a sh:Shape ;
>     sh:scopeClass  ex:Doctor, ex:Patience ;
>     sh:scopeNode ex:DrWho, ex:Bob
>     sh:scopeProperty ex:operated
>     sh:scopeInverseProperty ex:operated
>     sh:constraint [
>         sh:minInstanceCount 1 ;
>         sh:maxInstanceCount 1 ;
>     ] .
>
> do we require 1 doctor, 1 Patience, 1 ...
> or only 1 of all together?
>
> Also, what would be the definition when this constraint is referenced 
> from e.g. sh:shape?
> ex:MyHospitalShape
>    a sh:Shape
>    sh:scopeClass ex:Hospital
>    sh:property [
>        sh:predicate ex:personStayed
>        sh:shape ex:MyPersonShape
>   ]
>
> does the constraint apply an all hospitals with a property 
> ex:personStayed ( ex:Hospital[ex:stayed]) ?
>
> Best
> Dimitris
>
> On Mon, Jun 13, 2016 at 3:04 PM, RDF Data Shapes Working Group Issue 
> Tracker <sysbot+tracker@w3.org <mailto:sysbot+tracker@w3.org>> wrote:
>
>     shapes-ISSUE-168 (instance count): How to constrain number of
>     instances of a class in a graph [SHACL - Core]
>
>     http://www.w3.org/2014/data-shapes/track/issues/168
>
>     Raised by: Holger Knublauch
>     On product: SHACL - Core
>
>     There was recent discussion about how to specify min/max numbers
>     of instances in a graph.  Off-list I had also received a question
>     from schema.org <http://schema.org> people about how to state that
>     a graph/message must have exactly one instance of
>     schema:FlightReservation, possibly to ensure that a graph has a
>     starting point/root for validation.
>
>     A possible syntax that would easily work with the current
>     architecture could be
>
>     ex:MyShape
>         a sh:Shape ;
>         sh:scopeNode schema:FlightReservation ;
>         sh:constraint [
>             sh:minInstanceCount 1 ;
>             sh:maxInstanceCount 1 ;
>         ] .
>
>
>
>
>
>
> -- 
> Dimitris Kontokostas
> Department of Computer Science, University of Leipzig & DBpedia 
> Association
> Projects: http://dbpedia.org, http://rdfunit.aksw.org, 
> http://aligned-project.eu
> Homepage: http://aksw.org/DimitrisKontokostas
> Research Group: AKSW/KILT http://aksw.org/Groups/KILT
>