default_graph_snippet.js

// Comunica test: Query default graph with selected named graphs (excluding inference) using a clean wrapper
import { QueryEngine } from '@comunica/query-sparql';
import { DataFactory, Store } from 'n3';
const { namedNode, quad } = DataFactory;

// --- Custom RDFJS source wrapper ---
class FilteredDefaultGraphSource {
  constructor(store, defaultGraphIRIs) {
    this.store = store;
    this.defaultGraphIRIs = new Set(defaultGraphIRIs);
  }

  match(subject, predicate, object, graph) {
    // Merge only selected named graphs when querying the default graph
    if (!graph || graph.termType === 'DefaultGraph') {
      return Array.from(this.defaultGraphIRIs)
      .flatMap(g => this.store.getQuads(subject, predicate, object, g))
        [Symbol.iterator]();
    }
    // Pass through named graph queries
    return this.store.getQuads(subject, predicate, object, graph)[Symbol.iterator]();
  }
}

// --- Initialize engine and store ---
const engine = new QueryEngine();
const store = new Store();

// --- Add quads ---
// Asserted graphs
store.addQuad(quad(namedNode('http://example.org/a'), namedNode('http://example.org/p'), namedNode('http://example.org/asserted1'), namedNode('http://example.org/data1')));
store.addQuad(quad(namedNode('http://example.org/b'), namedNode('http://example.org/p'), namedNode('http://example.org/asserted2'), namedNode('http://example.org/data2')));
// Inference graphs
store.addQuad(quad(namedNode('http://example.org/a'), namedNode('http://example.org/p'), namedNode('http://example.org/inferred1'), namedNode('http://example.org/data1?inference')));
store.addQuad(quad(namedNode('http://example.org/b'), namedNode('http://example.org/p'), namedNode('http://example.org/inferred2'), namedNode('http://example.org/data2?inference')));

// --- Wrap store for filtered default graph ---
const defaultGraphIRIs = ['http://example.org/data1', 'http://example.org/data2'];
const filteredSource = new FilteredDefaultGraphSource(store, defaultGraphIRIs);

// --- SPARQL query ---
const query = `PREFIX ex: <http://example.org/>
SELECT ?s ?o WHERE { ?s ex:p ?o } ORDER BY ?s ?o`;

// --- Helper to run query ---
async function runQuery(description) {
  console.log(`\n=== ${description} ===`);
  const result = await engine.queryBindings(query, { sources: [filteredSource] });
  const bindings = await result.toArray();
  if (!bindings.length) console.log('(no results)');
  bindings.forEach(b => console.log(`- ${b.get('s').value} ${b.get('o').value}`));
}

// --- Execute ---
(async () => {
  await runQuery('Query default graph includes only asserted graphs (excludes inference)');
})();

// Outputs:
//
// === Query default graph includes only asserted graphs (excludes inference) ===
// - http://example.org/a http://example.org/asserted1
// - http://example.org/b http://example.org/asserted2