update changelog and add test for empty key

Author: bagerard

docs/changelog.rst

@@ -11,6 +11,7 @@ Development
   Typically to support mock mongo libraries like mongomock, montydb, mongita #2729
 - BREAKING CHANGE: connecting MongoEngine with mongomock should now use the new `mongo_client_class`
   For more info, check https://docs.mongoengine.org/guide/mongomock.html
+- Fix DictField that always gets marked as changed #2606
 
 Changes in 0.26.0
 =================

tests/document/test_delta.py

@@ -4,7 +4,7 @@
 
 from mongoengine import *
 from mongoengine.pymongo_support import list_collection_names
-from tests.utils import MongoDBTestCase
+from tests.utils import MongoDBTestCase, get_as_pymongo
 
 
 class TestDelta(MongoDBTestCase):
@@ -977,6 +977,24 @@ class MyDoc(Document):
         mydoc._clear_changed_fields()
         assert mydoc._get_changed_fields() == []
 
+    def test_delta_on_dict_empty_key_triggers_full_change(self):
+        """more of a bug (harmless) but empty key changes aren't managed perfectly"""
+
+        class MyDoc(Document):
+            dico = DictField()
+
+        MyDoc.drop_collection()
+
+        MyDoc(dico={"a": {"b": 0}}).save()
+
+        mydoc = MyDoc.objects.first()
+        assert mydoc._get_changed_fields() == []
+        mydoc.dico[""] = 3
+        assert mydoc._get_changed_fields() == ["dico"]
+        mydoc.save()
+        raw_doc = get_as_pymongo(mydoc)
+        assert raw_doc == {"_id": mydoc.id, "dico": {"": 3, "a": {"b": 0}}}
+
 
 if __name__ == "__main__":
     unittest.main()