Search filter on @optional property does not exclude results

[ddeboer]

Problem

When using $not (or other filters) on an @optional property in search criteria, the filter is placed inside an OPTIONAL block in the generated SPARQL. This means it doesn't exclude matching results — it only makes the optional binding unbound.

Example

Schema:

const schema = {
  '@type': 'dcat:Dataset',
  distribution: {
    '@id': 'dcat:distribution',
    '@array': true,
    '@schema': {
      '@type': 'dcat:Distribution',
      accessURL: 'dcat:accessURL',
      conformsTo: { '@id': 'dcterms:conformsTo', '@optional': true },
    },
  },
};

Search criteria:

await lens.find({
  where: {
    distribution: {
      conformsTo: { $not: 'https://www.w3.org/TR/rdf-sparql-query/' },
    },
  },
});

Generated SPARQL (actual)

OPTIONAL {
  ?iri_1 <dcterms:conformsTo> ?iri_1_1 .
  FILTER (?iri_1_1 != "https://www.w3.org/TR/rdf-sparql-query/") .
}

The FILTER is inside the OPTIONAL, so distributions with conformsTo = .../rdf-sparql-query/ still appear — their conformsTo value just becomes unbound.

Expected SPARQL

The filter should be moved outside the OPTIONAL block, using a !BOUND || != pattern:

OPTIONAL {
  ?iri_1 <dcterms:conformsTo> ?iri_1_1 .
}
FILTER (!BOUND(?iri_1_1) || ?iri_1_1 != "https://www.w3.org/TR/rdf-sparql-query/")

This would correctly exclude distributions whose conformsTo matches, while keeping distributions that have no conformsTo at all.

Version

LDkit 2.6.0