Error or NULL value when a reference is not found in the source data

[elsdvlee]

Suppose source data:

{
  "people": [
    {
      "name": "alice",
      "items": [
        {
          "type": "sword",
          "measures": {
            "weight": 1500,
            "length": 30
          }
        },
        {
          "type": "shield",
          "measures": {
            "weight": 2500
          }
        }
      ]
    },
    {
      "name": "bob",
      "items": [
        {
          "type": "flower",
          "measures" : {
            "weight": 15,
            "length" : 15
          }
        }
      ]
    }
  ]
}

and mapping:

@prefix rml: <http://w3id.org/rml/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix : <http://example.org/> .

:jsonSource a rml:LogicalSource ;
  rml:source [
    a rml:RelativePathSource , rml:Source ;
    rml:root rml:MappingDirectory ;
    rml:path "people.json" ;
  ] ;
  rml:referenceFormulation rml:JSONPath ;
  rml:iterator "$.people[*]" .

:jsonView a rml:LogicalView ;
  rml:viewOn :jsonSource ;
  rml:field [
    a rml:ExpressionField ;
    rml:fieldName "name" ;
    rml:reference "$.name" ;
  ] ;
  rml:field [
    a rml:IterableField ;
    rml:fieldName "item" ;
    rml:iterator "$.items[*]" ;
    rml:field [
      rml:fieldName "type" ;
      rml:reference "$.type" ;
    ] ;
    rml:field [
      a rml:IterableField ;
      rml:fieldName "measures";
      rml:iterator "$.measures";
      rml:field [
        a rml:ExpressionField ;
        rml:fieldName "length" ;
        rml:reference "$.length" ;
      ]
    ] ;
  ] .

:triplesMapItem a rml:TriplesMap ;
  rml:logicalSource :jsonView ;
  rml:subjectMap [
    rml:template "http://example.org/person/{name}/item/{item.type}" ;
  ] ;
  rml:predicateObjectMap [
    rml:predicate :haslength ;
    rml:objectMap [
      rml:reference "item.measures.length" ;
      rml:datatype xsd:integer ;
    ] ;
  ] .

Should the RML Engine throws an error because the length of the shield of alice is missing?
Or should it just not produce that triple?
In RML-Core, the RML Engine should throw an error when the reference is invalid, see RMLTC0023a-JSON.
@pmaria, @chrdebru, @bjdmeest, @dachafra What is your opinion? Is a jsonpath expression returning no results for some iterations the same as a missing column, also if it returns results for other iterations?

In the meanwhile, I will already correct RML LV 2c, where this field is not used in any triple maps and can/must be removed.

[elsdvlee]

Personnaly, I would prefer that a null value is produced in the Logical View and that the triple is skipped in the generation of the triples by the triples map.
So intermediate representation of the logical view:

#	name.#	name	item.#	item	item.measures.#	item.measures	item.measures.length.#	item.measures.length
0	0	alice	0	{"type": "sword", "measures": {"weight": 1500, "length": 30}}	0	{"weight": 1500, "length": 30}	0	30
0	0	alice	1	{"type": "shield", "measures": {"weight": 2500}}	0	{"weight": 2500}	null	null
1	0	bob	0	{"type": "flower", "measures": {"weight": 15, "length": 15}}	0	{"weight": 15, "length": 15}	0	15

And the output of the mapping:

<http://example.org/person/alice/item/sword> <http://example.org/hasLength> "30"^^<http://www.w3.org/2001/XMLSchema#integer> .
<http://example.org/person/bob/item/flower> <http://example.org/hasLength> "15"^^<http://www.w3.org/2001/XMLSchema#integer> .

I guess it depends on what should happen with an empty result sequence as output of a json reference for a specific logical iteration. How is this done in RML-Core?
I do understand that referencing a none existing column in a CSV file produces an error, but an empty result sequence from JSON path feels different to me.

[bjdmeest]

If we go back to R2RML: a reference to something non-existing produces an error.
We took that over in RML, and given kg-construct/rml-core#199 : yes, this should indeed currently produce an error.

A separate issue could be to introduce a 'undefined as NULL instead of error' flag that you could add to an abstract logical source (given this discussion keeps on coming back). I made that at kg-construct/rml-io-registry#46

[elsdvlee]

I will close this case, as test case 2c is already adapted so that is no longer impacted by the discussion of issue kg-construct/rml-io-registry#46