Fixed #21643 -- repeated execution of qs with F() + timedelta

Thanks Tim Graham for review.

django/db/models/sql/expressions.py

@@ -111,6 +111,7 @@ class SQLEvaluator(object):
     def evaluate_date_modifier_node(self, node, qn, connection):
         timedelta = node.children.pop()
         sql, params = self.evaluate_node(node, qn, connection)
+        node.children.append(timedelta)
 
         if (timedelta.days == timedelta.seconds == timedelta.microseconds == 0):
             return sql, params

tests/expressions_regress/tests.py

@@ -274,6 +274,13 @@ class FTimeDeltaTests(TestCase):
         self.days_long.append(e4.completed - e4.assigned)
         self.expnames = [e.name for e in Experiment.objects.all()]
 
+    def test_multiple_query_compilation(self):
+        # Ticket #21643
+        queryset = Experiment.objects.filter(end__lt=F('start') + datetime.timedelta(hours=1))
+        q1 = str(queryset.query)
+        q2 = str(queryset.query)
+        self.assertEqual(q1, q2)
+
     def test_delta_add(self):
         for i in range(len(self.deltas)):
             delta = self.deltas[i]