Processing tabular data from select queries

s-pipes-core/src/main/java/cz/cvut/spipes/util/QueryUtils.java

@@ -46,6 +46,11 @@ public static String nextResultsToValuesClause(ResultSet resultSet, int rowsCoun
     }
 
     public static String serializeToSparql(RDFNode rdfNode) {
+
+        if (rdfNode == null) {
+            return "UNDEF";
+        }
+
         ParameterizedSparqlString pss= new ParameterizedSparqlString();
         pss.appendNode(rdfNode);
         return pss.toString();

s-pipes-core/src/main/java/cz/cvut/spipes/util/query/OneStepBackExtendedResultSet.java

@@ -0,0 +1,102 @@
+package cz.cvut.spipes.util.query;
+
+import org.apache.jena.query.QuerySolution;
+import org.apache.jena.query.QuerySolutionMap;
+import org.apache.jena.query.ResultSet;
+import org.apache.jena.rdf.model.Model;
+import org.apache.jena.sparql.engine.binding.Binding;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * ResultSet that extends the original result set by adding bindings from the previous query solution.
+ * The previous query solution is available under the same variable name with suffix "__previous".
+ *
+ * <p>Example use:</p>
+ * <pre>
+ *     ResultSet resultSet = ...;
+ *     ResultSet extendedResultSet = new OneStepBackExtendedResultSet(resultSet);
+ *     while (extendedResultSet.hasNext()) {
+ *         QuerySolution querySolution = extendedResultSet.next();
+ *         RDFNode currentResource = querySolution.get("resource");
+ *         RDFNode previousResource = querySolution.get("resource__previous");
+ *     }
+ *     </pre>
+ */
+public class OneStepBackExtendedResultSet implements ResultSet  {
+
+    private static final String PREVIOUS_BINDING_SUFFIX = "__previous";
+
+    private final ResultSet resultSet;
+    private final List<String> resultVars;
+
+    private QuerySolution previousQuerySolution;
+
+    public OneStepBackExtendedResultSet(ResultSet resultSet) {
+        this.resultSet = resultSet;
+        this.previousQuerySolution = new QuerySolutionMap();
+        this.resultVars = getExtendedResultVars(resultSet.getResultVars());
+    }
+
+    private static List<String> getExtendedResultVars(List<String> originalResultVars) {
+        originalResultVars.stream()
+            .filter(s -> s.endsWith(PREVIOUS_BINDING_SUFFIX))
+            .findAny()
+            .ifPresent(s -> {
+                throw new IllegalArgumentException(
+                    "The result set already contains a variable with suffix " + PREVIOUS_BINDING_SUFFIX
+                );
+            });
+        List<String> joinedList = new ArrayList<String>();
+        joinedList.addAll(originalResultVars);
+        joinedList.addAll(originalResultVars.stream()
+            .map(v -> v + PREVIOUS_BINDING_SUFFIX)
+            .toList());
+        return joinedList;
+    }
+
+    @Override
+    public boolean hasNext() {
+        return resultSet.hasNext();
+    }
+
+    @Override
+    public QuerySolution next() {
+        QuerySolution nextQuerySolution = resultSet.next();
+
+        QuerySolutionMap querySolution = new QuerySolutionMap();
+        querySolution.addAll(nextQuerySolution);
+        previousQuerySolution.varNames().
+            forEachRemaining(
+            varName -> querySolution.add(varName + PREVIOUS_BINDING_SUFFIX, previousQuerySolution.get(varName))
+        );
+        previousQuerySolution = nextQuerySolution;
+        return querySolution;
+    }
+
+    @Override
+    public QuerySolution nextSolution() {
+        return this.next();
+    }
+
+    @Override
+    public Binding nextBinding() {
+        throw new UnsupportedOperationException("Not implemented");
+    }
+
+    @Override
+    public int getRowNumber() {
+        return resultSet.getRowNumber();
+    }
+
+    @Override
+    public List<String> getResultVars() {
+        return resultVars;
+    }
+
+    @Override
+    public Model getResourceModel() {
+        return resultSet.getResourceModel();
+    }
+}

s-pipes-modules/module-sparql-endpoint/src/main/java/cz/cvut/spipes/modules/ApplyConstructWithChunkedValuesAndScrollableCursorModule.java

@@ -18,6 +18,10 @@
 import org.topbraid.spin.model.Select;
 
 /**
+ * Apply construct query with chunked values and scrollable cursor.
+ * See {@link ApplyConstructWithChunkedValuesModule} and {@link ApplyConstructWithScrollableCursorModule}
+ * for more details.
+ *
  * TODO Order of queries is not enforced.
  * TODO issue with redundant call {@link ScrollableCursorProvider}
  * TODO supports only one CONSTRUCT query

s-pipes-modules/module-sparql-endpoint/src/main/java/cz/cvut/spipes/modules/ApplyConstructWithChunkedValuesModule.java

@@ -2,9 +2,9 @@
 
 import cz.cvut.spipes.constants.KBSS_MODULE;
 import cz.cvut.spipes.constants.SML;
-import cz.cvut.spipes.engine.VariablesBinding;
 import cz.cvut.spipes.modules.annotations.SPipesModule;
 import cz.cvut.spipes.util.QueryUtils;
+import cz.cvut.spipes.util.query.OneStepBackExtendedResultSet;
 import lombok.extern.slf4j.Slf4j;
 import org.apache.jena.query.*;
 import org.apache.jena.rdf.model.Model;
@@ -16,13 +16,41 @@
 import java.util.Objects;
 
 /**
- * TODO Order of queries is not enforced.
+ * Apply construct query with chunked values.
+ * The construct query is provided as a template with marker `#${VALUES}`.
+ * The marker is substituted with chunked values from a select query.
+ *
+ * <p>Example select query:</p>
+ * <pre>
+ * SELECT ?person ?lastName
+ * WHERE {
+ *   ?person a foaf:Person .
+ *   ?person foaf:lastName ?lastName .
+ * }
+ * </pre>
+ *
+ * <p>Example construct query:</p>
+ * <pre>
+ * CONSTRUCT {
+ *   ?person a :User .
+ *   ?person :has-last-name ?lastName .
+ * }
+ * WHERE {
+ *   #${VALUES}
+ *   FILTER(strlen(?lastName) < 20)
+ * }
+ * </pre>
+ *
+ * <p>TODOs:</p>
+ * <ul>
+ *   <li>Order of queries is not enforced.</li>
+ * </ul>
  */
 @Slf4j
 @SPipesModule(label = "apply construct with chunked values", comment = "Apply construct with chunked values.")
 public class ApplyConstructWithChunkedValuesModule extends ApplyConstructAbstractModule {
 
-    private static final String TYPE_URI = KBSS_MODULE.uri;
+    private static final String TYPE_URI = KBSS_MODULE.uri + "apply-construct-with-chunked-values";
     private static final String TYPE_PREFIX = TYPE_URI + "/";
     private static final int DEFAULT_CHUNK_SIZE = 10;
     private static final String VALUES_CLAUSE_MARKER_NAME = "VALUES";
@@ -68,9 +96,7 @@ public void initializeQuery() {
 
         log.debug("Executing query of chunk provider ...");
 
-        selectResultSet = execution.execSelect();
-
-        VariablesBinding variablesBinding = new VariablesBinding();
+        selectResultSet = new OneStepBackExtendedResultSet(execution.execSelect());
 
         if (! selectResultSet.hasNext()) {
             log.debug("\"{}\" query did not return any values.", getLabel());
@@ -103,7 +129,7 @@ protected String substituteQueryMarkers(int currentIteration, String queryStr) {
 
         String markerValue = QueryUtils.nextResultsToValuesClause(getCurrentResultSetInstance(), chunkSize);
 
-        log.debug("Creating query with values clause: \n{}.", markerValue );
+        log.trace("Creating query with values clause: \n{}.", markerValue );
 
         return QueryUtils
             .substituteMarkers(VALUES_CLAUSE_MARKER_NAME,