NG4J - Named Graph API for Jena

Hey,

On Thursday 19 November 2009 08:57:26 Arun Rao wrote:
> Hi,
> 
> I am trying to create an InfModel object using a NG4J NamedGraphModel and
> when I get the raw model from the InfModel, I expect to get back the
> original NamedGraphModel.

So? Do you get it?
(i.e. is infModel.getRawModel() an instance of NamedGraphModel?)

> While this comparison returns true:
> infModel.getRawModel().equals(ngModel)
> 
> I am not able to cast infModel.getRawModel().getGraph() to NamedGraphSet
> since infModel.getRawModel().getGraph() seems to be an instance of an inner
> class of NamedGraphSetImpl.

That's true.

> There does not seem to be a simple way in which I can get NamedGraphModel
> back.

I would expect you get the NamedGraphModel by calling getRawModel() - at least 
that's what the Jena Javadoc says.

The problem seems to be (I assume based on your use of the getGraph() method) 
that you are not interested in the NamedGraphModel that backs the infModel but 
you are interested in the NamedGraphSet that backs the NamedGraphModel. Right?
If so, you should be able to call 

( (NamedGraphModel) infModel.getRawModel() ).getNamedGraphSet()

Greetings,
Olaf

Hi Olaf,

On Thu, Nov 19, 2009 at 2:59 PM, Olaf Hartig <hartig@...
> wrote:

> Hey,
>
> On Thursday 19 November 2009 08:57:26 Arun Rao wrote:
> > Hi,
> >
> > I am trying to create an InfModel object using a NG4J NamedGraphModel and
> > when I get the raw model from the InfModel, I expect to get back the
> > original NamedGraphModel.
>
> So? Do you get it?
> (i.e. is infModel.getRawModel() an instance of NamedGraphModel?)
>

No, and that's the issue.

Here is some code:
System.out.println("rawModel instanceof NamedGraphModel?: ");
System.out.println(infModel.getRawModel() instanceof NamedGraphModel);
NamedGraphSet ngSet = ((NamedGraphModel)
infModel.getRawModel()).getNamedGraphSet();

Output:
rawModel instanceof NamedGraphModel?:
false
java.lang.ClassCastException: com.hp.hpl.jena.rdf.model.impl.ModelCom cannot
be cast to de.fuberlin.wiwiss.ng4j.NamedGraphModel
    at com.test.NG4JTest.main(NG4JTest.java:52)


>
> > While this comparison returns true:
> > infModel.getRawModel().equals(ngModel)
> >
> > I am not able to cast infModel.getRawModel().getGraph() to NamedGraphSet
> > since infModel.getRawModel().getGraph() seems to be an instance of an
> inner
> > class of NamedGraphSetImpl.
>
> That's true.
>
> > There does not seem to be a simple way in which I can get NamedGraphModel
> > back.
>
> I would expect you get the NamedGraphModel by calling getRawModel() - at
> least
> that's what the Jena Javadoc says.
>
> The problem seems to be (I assume based on your use of the getGraph()
> method)
> that you are not interested in the NamedGraphModel that backs the infModel
> but
> you are interested in the NamedGraphSet that backs the NamedGraphModel.
> Right?
> If so, you should be able to call
>
> ( (NamedGraphModel) infModel.getRawModel() ).getNamedGraphSet()
>

Frankly, either would do. All I want is a way to get back the provenance
information.


>
> Greetings,
> Olaf
>
>
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Thanks,
Arun

Hey,

On Friday 20 November 2009 06:50:37 Arun Rao wrote:
> [...]
> > > I am trying to create an InfModel object using a NG4J NamedGraphModel
> > > and when I get the raw model from the InfModel, I expect to get back
> > > the original NamedGraphModel.
> >
> > So? Do you get it?
> > (i.e. is infModel.getRawModel() an instance of NamedGraphModel?)
> 
> No, and that's the issue.
> 
> Here is some code:
> System.out.println("rawModel instanceof NamedGraphModel?: ");
> System.out.println(infModel.getRawModel() instanceof NamedGraphModel);
> NamedGraphSet ngSet = ((NamedGraphModel)
> infModel.getRawModel()).getNamedGraphSet();
> 
> Output:
> rawModel instanceof NamedGraphModel?:
> false
> java.lang.ClassCastException: com.hp.hpl.jena.rdf.model.impl.ModelCom
>  cannot be cast to de.fuberlin.wiwiss.ng4j.NamedGraphModel
>     at com.test.NG4JTest.main(NG4JTest.java:52)

Okay, after digging a bit into the Jena code I see the problem. Their 
implementation of the InfModel interface does not store the base model but it 
stores the graph of the base model (i.e. the object returned by the 'getGraph' 
method of the base model). Each call to 'getRawModel()' creates a new model 
from the stored base graph.
The problem is that 'getGraph' of a NamedGraphModel returns a union of all 
named graphs in the NamedGraphSet the NamedGraphModel is based on. The class 
that represents this union is an internal class -as you already experienced- 
for which there is no possibility to come back to the NamedGraphSet.
Hence, unfortunately, the only possibility to get back the original 
NamedGraphSet is to keep it seperately. You could create your own InfModel 
class (derived from Jena's InfModelImpl) and keep the reference to the 
NamedGraphSet there.

Olaf