Update changelog and add another ut case

docs/changelog.rst

@@ -6,6 +6,7 @@ Development
 ===========
 - (Fill this out as you fix issues and develop your features).
 - Fix .only() working improperly after using .count() of the same instance of QuerySet
+- POTENTIAL BREAKING CHANGE: All result fields are now passed, including internal fields (_cls, _id) when using `QuerySet.as_pymongo` #1976
 
 =================
 Changes in 0.16.3

tests/queryset/queryset.py

@@ -4709,19 +4709,22 @@ class QuerySetTest(unittest.TestCase):
 
         serialized_user = User.objects.exclude(
             'password_salt', 'password_hash').as_pymongo()[0]
-        self.assertEqual(set(['_id', 'email']), set(serialized_user.keys()))
+        self.assertEqual({'_id', 'email'}, set(serialized_user.keys()))
 
         serialized_user = User.objects.exclude(
             'id', 'password_salt', 'password_hash').to_json()
         self.assertEqual('[{"email": "ross@example.com"}]', serialized_user)
 
+        serialized_user = User.objects.only('email').as_pymongo()[0]
+        self.assertEqual({'_id', 'email'}, set(serialized_user.keys()))
+
         serialized_user = User.objects.exclude(
             'password_salt').only('email').as_pymongo()[0]
-        self.assertEqual(set(['_id', 'email']), set(serialized_user.keys()))
+        self.assertEqual({'_id', 'email'}, set(serialized_user.keys()))
 
         serialized_user = User.objects.exclude(
             'password_salt', 'id').only('email').as_pymongo()[0]
-        self.assertEqual(set(['email']), set(serialized_user.keys()))
+        self.assertEqual({'email'}, set(serialized_user.keys()))
 
         serialized_user = User.objects.exclude(
             'password_salt', 'id').only('email').to_json()