Describe query's result

[ahka01]

Hi everybody,

I have the following question related to using DESCRIBE query:

how could I make the query describe also the objects that are blank or unnamed node?

Considering the following simple tripels:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix ex: <http://example.com> .

ex:a a ex:b;
    ex:c ex:d;
    ex:e [
        a ex:f;
        ex:a ex:m
    ];
.

The results to describe the resource ex:a looks like:

<http://example.coma> a <http://example.comb>;
    <http://example.comc> <http://example.comd>;
    <http://example.come> _:bc_0_n3-02.

What I wish it to have is to return also what does _:bc_0_n3-02 contains. Is there a way to achieve that?

PS: Fuseki returns the description of objects of triples if it is from kind blank nodes as well.

Thanks a lot in advance for answering and for the great job of comunica :)

Best regards

[rubensworks]

The problem with DESCRIBE is that it's not very strictly defined in the SPARQL spec. So executing the same DESCRIBE query in different engines may produce different results.

My recommendation would be to use CONSTRUCT queries instead, as you can specifically define the template of triples you want to produce.

Using a CONSTRUCT WHERE, and FILTER to check for blank nodes, it should be possible to drill down on blank node entities. (I don't have a query at hand for this though)

[ahka01]

Thanks a lot for the hint

[ahka01]

applying the following DESCRIBE query:

DESCRIBE ?o {
    ?s ?p ?o .
        FILTER(isBlank(?o)) 
}

on these dummy triples:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix ex: <http://example.com/> .

ex:a a ex:b;
    ex:c ex:d;
    ex:e [
        a ex:f;
        ex:v ex:m ;
    ]
.

I'm getting the following results:

subject                      predicate                                                                          object                                                                                         
----------------------------------------------------------------------------------------------------------------------------
bc_0_n3-00            http://www.w3.org/1999/02/22-rdf-syntax-ns#type     http://example.com/f                                                                                 
bc_0_n3-01             http://example.com/v                                                       http://example.com/m  

I'm wondering why the the subjects here have different labels? They shouldn't be the same?

[jacoscaz]

Where is http://example.com/v coming from? It's not in the triples above if I am not mistaken.

[ahka01]

I edited the comment again by adding the triples, that I'm referring to. They are indeed different from the one in the main question. Sorry for that!

[jacoscaz]

@ahka01 thanks for updating!

I don't have the time to look further into this today but this looks like a bug having something to do with blank node correlation. @rubensworks I have indeed done some work on this topic as a fix to jacoscaz/quadstore#134 . This comment lists the relevant upstream commits.

[jacoscaz]

This comment might also be relevant here (or it might not, approach with caution as with everything related to blank nodes):

The spec says that "blank node labels are scoped to the template for each solution". Since there are 4 solutions here, there are 4 template instantiations, each with their own (non-clashing) bnode.

Perhaps there's something that's triggering a new template instantiation per each quad even in this case?

[rubensworks]

Perhaps there's something that's triggering a new template instantiation per each quad even in this case?

Yeah, it might be related to that.

@ahka01 If you would, you could open a bug report and/or PR with a fix. In any case, I think using CONSTRUCT queries might be a better solution going forward.