Annotated count fix index error (#1707)

* Fix for annotated count

* updated changelog

* added tests to show what the fix is about

CHANGELOG.rst

@@ -8,6 +8,9 @@ Changelog
 
 0.21
 ====
+0.21.7
+------
+- Fix bug when using annotate and count at the same time but the annotation does not match anything, leading to an IndexError
 
 0.21.6
 ------

tests/test_aggregation.py

@@ -236,3 +236,10 @@ async def test_count_after_aggregate_m2m(self):
 
         res = await query.count()
         assert res == 2
+
+    async def test_count_without_matching(self) -> None:
+        await Tournament.create(name="Test")
+
+        query = Tournament.annotate(events_count=Count("events")).filter(events_count__gt=0).count()
+        result = await query
+        assert result == 0

tortoise/queryset.py

@@ -1335,6 +1335,8 @@ def __await__(self) -> Generator[Any, None, int]:
 
     async def _execute(self) -> int:
         _, result = await self._db.execute_query(str(self.query))
+        if not result:
+            return 0
         count = list(dict(result[0]).values())[0] - self.offset
         if self.limit and count > self.limit:
             return self.limit