remove stale threads after all comments have been deleted

isso/db/__init__.py

@@ -20,6 +20,13 @@ class SQLite3:
         self.threads = Threads(self)
         self.comments = Comments(self)
 
+        self.execute([
+            'CREATE TRIGGER IF NOT EXISTS remove_stale_threads',
+            'AFTER DELETE ON comments',
+            'BEGIN',
+            '    DELETE FROM threads WHERE id NOT IN (SELECT tid FROM comments);',
+            'END'])
+
     def execute(self, sql, args=()):
 
         if isinstance(sql, (list, tuple)):

specs/test_comment.py

@@ -151,12 +151,14 @@ class TestComments(unittest.TestCase):
         r = client.delete('/id/1')
         assert r.status_code == 200
         assert loads(r.data)['mode'] == 4
+        assert '/path/' in self.app.db.threads
 
         assert self.get('/?uri=%2Fpath%2F&id=1').status_code == 200
         assert self.get('/?uri=%2Fpath%2F&id=2').status_code == 200
 
         r = client.delete('/id/2')
         assert self.get('/?uri=%2Fpath%2F').status_code == 404
+        assert '/path/' not in self.app.db.threads
 
     def testDeleteWithMultipleReferences(self):
         """
@@ -270,6 +272,13 @@ class TestComments(unittest.TestCase):
         self.put('/id/1', data=json.dumps({"text": "Typo"}))
         assert loads(self.get('/id/1').data)["text"] == "<p>Typo</p>\n"
 
+    def testDeleteCommentRemovesThread(self):
+
+            rv = self.client.post('/new?uri=%2F', data=json.dumps({"text": "..."}))
+            assert '/' in self.app.db.threads
+            self.client.delete('/id/1')
+            assert '/' not in self.app.db.threads
+
 
 class TestModeratedComments(unittest.TestCase):