Fix too early StopIteration on empty result sets

Fixes #77, thanks to @hfaran for report

dataset/persistence/util.py

@@ -24,10 +24,8 @@ class ResultIter(object):
         if not isgenerator(result_proxies):
             result_proxies = iter((result_proxies, ))
         self.result_proxies = result_proxies
-
         self.count = 0
-        if not self._next_rp():
-            raise StopIteration
+        self.rp = None
 
     def _next_rp(self):
         try:
@@ -39,6 +37,9 @@ class ResultIter(object):
             return False
 
     def __next__(self):
+        if self.rp is None:
+            if not self._next_rp():
+                raise StopIteration
         row = self.rp.fetchone()
         if row is None:
             if self._next_rp():

test/test_persistence.py

@@ -275,3 +275,8 @@ class TableTestCase(unittest.TestCase):
         keys = list(res.next().keys())
         assert keys[0] == 'temperature'
         assert keys[1] == 'place'
+
+    def test_empty_query(self):
+        m = self.tbl.find(place='not in data')
+        l = list(m)  # exhaust iterator
+        assert len(l) == 0