Wrong implementation of JsonPath

[Sealdolphin]

Hi,

I recently came across a possible bug:

My JSON architecture looks like this:

{
  "elementStore": [
    {
      "type" : "type1",
      "elements": [
        {
          "ord": "first"
        },
    {
          "ord": "second"
        },
        {
          "ord": "third"
        }
      ]
    },
{
      "type": "type2",
      "elements": [
        {
          "ord": "first2"
        },
    {
          "ord": "second2"
        },
        {
          "ord": "third2"
        }
      ]
    }
}

And I tried to extract the different element ordinals (ord) with the help of JSONPath. My RML fragment:

<#elementsMap> a rr:TriplesMap;

    rml:logicalSource [ 
        rml:source "<my JSON file>";
        rml:referenceFormulation ql:JSONPath;
        rml:iterator "$.elementStore[*]";
    ];

    rr:subjectMap [ 
        rr:template "/{.elements[*].ord}";
        rr:class mm:ordinal;
    ];

I expected that the result should include all three elements however, I only got the first one

[pheyvaer]

Hi @Sealdolphin

Can you explain what the expected result is in terms of RDF?

[Sealdolphin]

Hi, yes, although I think I found the problem in the source code.

Expected output:

<http://example.com/first> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.com#ordinal>.
<http://example.com/second> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.com#ordinal>.
<http://example.com/third> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.com#ordinal>.
<http://example.com/first2> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.com#ordinal>.
<http://example.com/second2> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.com#ordinal>.
<http://example.com/third2> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.com#ordinal>.

Actual output:

<http://example.com/first> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.com#ordinal>.
<http://example.com/first2> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.com#ordinal>.

UPDATE: I looked it again, and it's not the line I first linked.. linking again

[Sealdolphin]

@pheyvaer is it intended behavior, or should it return all possible JSONPath records as subjects?

[pheyvaer]

I would expect the RML to be something like this to get the RDF you want

<#elementsMap> a rr:TriplesMap;

    rml:logicalSource [ 
        rml:source "<my JSON file>";
        rml:referenceFormulation ql:JSONPath;
        rml:iterator "$.elementStore[*].elements[*]";
    ];

    rr:subjectMap [ 
        rr:template "{ord}";
        rr:class mm:ordinal;
    ];

And this because your subjects are in the elements array and not in the elementStore array. Well at least not directly.

[Sealdolphin]

I would expect the RML to be something like this to get the RDF you want

Thanks for the fast response. This mapping does work indeed, however this way I cannot access any other property of the objects in the elementStore (besides elements). Is there a way to make my mapping work, or do I want to use the RMLMapper inherently the wrong way?

I thought that while the JSONPath evaluator parses my JSONPath correctly, that would be the same result in the case of here.

UPDATE: To be precise, my problem is, that JSONPath does not have ancestor operator and with RMLMapper I can only use previously defined subjects as objects (with parentTriplesMap), but not the other way around. So unless I change my whole data structure, I cannot read it the RMLMapper, or can I?

[pheyvaer]

Hi @Sealdolphin

Yeah, the problem is that JSONPath is limited in that regard and at the moment there is not other reference formulation available for JSON and thus also not implemented in the RMLMapper. Preprocessing the JSON structure to work around this issue is the best option at the moment. We have done this too before in the past.

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