Merge "Fix the return of statistic with getting no sample"

ceilometer/storage/impl_sqlalchemy.py

@@ -735,6 +735,10 @@ class Connection(base.Connection):
             res = self._make_stats_query(sample_filter,
                                          None,
                                          aggregate).first()
+            if not res:
+                # NOTE(liusheng):The 'res' may be NoneType, because no
+                # sample has found with sample filter(s).
+                return
 
         query = self._make_stats_query(sample_filter, groupby, aggregate)
         # HACK(jd) This is an awful method to compute stats by period, but

ceilometer/tests/storage/test_storage_scenarios.py

@@ -1359,6 +1359,14 @@ class StatisticsTest(DBTestBase,
         self.assertEqual(results.sum, 18)
         self.assertEqual(results.avg, 6)
 
+    def test_with_no_sample(self):
+        f = storage.SampleFilter(
+            user='user-not-exists',
+            meter='volume.size',
+        )
+        results = list(self.conn.get_meter_statistics(f, period=1800))
+        self.assertEqual([], results)
+
 
 class StatisticsGroupByTest(DBTestBase,
                             tests_db.MixinTestsWithBackendScenarios):