Optional keyword does not seem to work

[pietervdvn]

Issue type:

• ❓ Question

---

Question:

I have two json-lds. They both graphs, each containing a geocoordinate (with schema.org/geo). Optionally, they might also contain a 'shape' property defining a Polygon.

I'm trying to extract the shape if there is one with a comunica sparql query, which is more or less:

SELECT * WHERE {
?parking a <http://schema.mobivoc.org/BicycleParkingStation>.
        GRAPH ?g {
            VALUES ?type { vpt:BicycleLocker vpt:BicycleStand vpt:ResidentBicycleParking vpt:PublicBicycleParking }.
            ?section a ?type.
            
            OPTIONAL {
                ?section schema:geo [
                    a schema:GeoCoordinates ;
                    schema:latitude ?latitude ;
                    schema:longitude ?longitude
                ] }.
            OPTIONAL {
                ?x schema:geo [
                a schema:GeoShape ;
                schema:polygon ?shape
                ]
            }.
       }
}

However, this result does not give any bindings if there is no geoshape present - even though this is wrapped into an optional.

When using the link-traversal-build with lenient: true as option, this suddenly works.

You can reproduce this issue; I've made a (more or less) minimal example here:
https://github.com/pietervdvn/sparql-test

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[rubensworks]

I see you're using Comunica v3. Could you try it out on the latest v2 version (2.10.2) to check if the problem already existed back then? (might be a regression in v3)

[pietervdvn]

I used a 2.10 version previously, which had the same symptoms.

[rubensworks]

I see. Very surprising that it does work with link traversal though.

Could you check what happens if you move the data to the default graph and remove the GRAPH keyword from the query? Perhaps that's related as well.

[pietervdvn]

I added a version without graph-embedding in my minimal test case: https://github.com/pietervdvn/sparql-test

This one works and gives results as expected.

However, I'm reusing data from an upstream provider where I cannot really change the shape of the data they provide...

[rubensworks]

Hmm, that's interesting.
Could you try querying the graph-based data in a Turtle format? Because I somehow expect it having to do with JSON-LD parsing, and not the actual query processor.

[pietervdvn]

I'm not really sure how to convert this efficiently to TTL. This website ignored everything within the graph.

However, I did a simple test query ( ?s ?p ?. GRAPH ?g { ?sg ?pg ?og }) and all the expected values are there.