Fix issue w/model_to_dict when only aliases present.

Fixes #2821

playhouse/shortcuts.py

@@ -39,6 +39,7 @@ def model_to_dict(model, recurse=True, backrefs=False, only=None,
     should_skip = lambda n: (n in exclude) or (only and (n not in only))
 
     if fields_from_query is not None:
+        only.add('__sentinel__')  # Add a placeholder to make non-empty.
         for item in fields_from_query._returning:
             if isinstance(item, Field):
                 only.add(item)

tests/shortcuts.py

@@ -425,6 +425,18 @@ def test_fields_from_query(self):
                              {'magic': 1337, 'content': 'u0-0',
                               'user': {'username': 'u0'}})
 
+    def test_fields_from_query_alias(self):
+        q = User.select(User.username.alias('name'))
+        res = q[0]
+        self.assertEqual(model_to_dict(res, fields_from_query=q),
+                         {'name': 'peewee'})
+
+        UA = User.alias()
+        q = UA.select(UA.username.alias('name'))
+        res = q[0]
+        self.assertEqual(model_to_dict(res, fields_from_query=q),
+                         {'name': 'peewee'})
+
     def test_only_backref(self):
         for i in range(3):
             Tweet.create(user=self.user, content=str(i))