Redefine random nodes query template to sample directly (#449 #501)

frontend/src/sparql/compile-query.ts

@@ -1,5 +1,5 @@
 import Model from '../core/model';
-import { nsTable, nsMap } from '../common-rdf/ns';
+import { source, item, nsTable, nsMap } from '../common-rdf/ns';
 import userChannel from '../common-user/user-radio';
 
 import itemsTemplate from './query-templates/items-for-source-template';
@@ -52,13 +52,9 @@ export function nodesByUserQuery(
 }
 
 export function randomNodesQuery(
-    randomNodes: Model[], lastNode: Model, options: SPARQLQueryOptions = {}
+    itemQuery: boolean, options: SPARQLQueryOptions = { limit: 10 }
 ) {
-    const data = {
-        ...defaultOptions,
-        randomNodes: randomNodes.map( model => model.get('value')),
-        lastNode: lastNode.get('value'),
-        ...options,
-    };
+    const nsLength = (itemQuery ? source() : item()).length;
+    const data = { ...defaultOptions, itemQuery, nsLength, ...options };
     return randomNodesTemplate(data);
 }

frontend/src/sparql/query-templates/random-nodes-template.hbs

@@ -1,7 +1,24 @@
-PREFIX dcterms: <http://purl.org/dc/terms/>
-PREFIX oa: <http://www.w3.org/ns/oa#>
-CONSTRUCT {?node ?p ?o}
-WHERE {
-    ?node ?p ?o.
-    FILTER ({{#each randomNodes}}?node = <{{this}}> ||{{/each}} ?node = <{{lastNode}}>)
+{{>sparqlPreamble}}
+
+construct {
+    ?subject ?prop ?val
+} where {
+    ?subject ?prop ?val
+    {
+        select distinct ?subject where {
+            {{#itemQuery}}
+            ?anno oa:hasBody ?subject.
+            ?subject a ?x.
+            {{/itemQuery}}
+            {{^itemQuery}}
+            ?subject a vocab:Source.
+            {{/itemQuery}}
+            bind (substr(str(?subject), {{nsLength}}) as ?serial)
+            bind (strlen(?serial) as ?cap)
+            bind (substr(?serial, ?cap) as ?suffix)
+            bind (substr(?serial, 1, ?cap - 1) as ?prefix)
+        }
+        order by md5(concat(?suffix, substr(str(rand()), 2), ?prefix))
+        limit {{limit}}
+    }
 }