[Bug #439] Support ordering by entity identifier in SOQL.

jopa-impl/src/main/antlr4/cz/cvut/kbss/jopa/query/soql/Soql.g4

@@ -133,7 +133,7 @@ functionsReturningBoolean
 
 orderByClause: ORDER BY orderByItem (',' orderByItem)* ;
 
-orderByItem: singleValuedObjectPathExpression (ASC | DESC)? ;
+orderByItem: (singleValuedObjectPathExpression | simpleSubpath) (ASC | DESC)? ;
 
 groupByClause: GROUP BY groupByItem (',' groupByItem)* ;
 

jopa-impl/src/main/java/cz/cvut/kbss/jopa/query/soql/SoqlOrderParameter.java

@@ -37,6 +37,10 @@ public void setAttribute(SoqlAttribute attribute) {
     }
 
     public String getOrderByPart(String rootVariable) {
+        if (getAsParam(rootVariable).equals(rootVariable)) {
+            // Ordering by the entity identifier
+            return SoqlConstants.ASC.equals(orderingBy) ? rootVariable + " " : orderingBy + "(" + rootVariable + ") ";
+        }
         String param = attribute.requiresFilter() ? getAsParam(rootVariable).substring(1) : attribute.getValue().substring(1);
         return orderingBy + "(?" + param + ") ";
     }

jopa-impl/src/main/java/cz/cvut/kbss/jopa/query/soql/SoqlQueryListener.java

@@ -437,7 +437,9 @@ private SoqlNode linkObjectPathExpression(ParserRuleContext ctx) {
         }
         setIris(firstNode);
         if (currentNode.getIri().isEmpty()) {
-            currentNode.getParent().clearChildren();
+            if (currentNode.getParent() != null) {
+                currentNode.getParent().clearChildren();
+            }
             this.isInObjectIdentifierExpression = true;
         }
         return firstNode;

jopa-impl/src/test/java/cz/cvut/kbss/jopa/query/soql/SoqlQueryParserTest.java

@@ -999,4 +999,24 @@ void parseQueryHandlesVariableCorrespondingToEntityIdentifier() {
                 " . ?personUri " + strUri(Vocabulary.p_p_username) + " ?username . }";
         parseAndAssertEquality(expectedSparql, soql);
     }
+
+    /**
+     * Bug #439
+     */
+    @Test
+    void parseQuerySupportsOrderingByEntityIdentifier() {
+        final String soql = "SELECT p FROM Person p ORDER BY p.uri";
+        final String expectedSparql = "SELECT ?x WHERE { ?x a " + strUri(Vocabulary.c_Person) + " . } ORDER BY ?x";
+        parseAndAssertEquality(expectedSparql, soql);
+    }
+
+    /**
+     * Bug #439 - alternative syntax
+     */
+    @Test
+    void parseQuerySupportsOrderingByEntity() {
+        final String soql = "SELECT p FROM Person p ORDER BY p";
+        final String expectedSparql = "SELECT ?x WHERE { ?x a " + strUri(Vocabulary.c_Person) + " . } ORDER BY ?x";
+        parseAndAssertEquality(expectedSparql, soql);
+    }
 }