Fix bug when using GenericReferenceField, modifications to the referenced document are tracked in the parent #1934

docs/changelog.rst

@@ -6,6 +6,8 @@ Development
 ===========
 - QuerySet limit function behaviour: Passing 0 as parameter will return all the documents in the cursor #1611
 - bulk insert updates the ids of the input documents #1919
+- Fix an harmless bug related to GenericReferenceField where modifications in the generic-referenced document
+    were tracked in the parent (#1934)
 - (Fill this out as you fix issues and develop your features).
 =======
 Changes in 0.15.4

mongoengine/base/document.py

@@ -528,6 +528,7 @@ class BaseDocument(object):
         EmbeddedDocument = _import_class('EmbeddedDocument')
         DynamicEmbeddedDocument = _import_class('DynamicEmbeddedDocument')
         ReferenceField = _import_class('ReferenceField')
+        GenericReferenceField = _import_class('GenericReferenceField')
         SortedListField = _import_class('SortedListField')
 
         changed_fields = []
@@ -560,7 +561,7 @@ class BaseDocument(object):
             elif (isinstance(data, (list, tuple, dict)) and
                     db_field_name not in changed_fields):
                 if (hasattr(field, 'field') and
-                        isinstance(field.field, ReferenceField)):
+                        isinstance(field.field, (ReferenceField, GenericReferenceField))):
                     continue
                 elif isinstance(field, SortedListField) and field._ordering:
                     # if ordering is affected whole list is changed

tests/fields/fields.py

@@ -2816,7 +2816,32 @@ class FieldTest(MongoDBTestCase):
         doc = Doc.objects.get(ref=DBRef('doc', doc1.pk))
         self.assertEqual(doc, doc2)
 
-    def test_generic_reference_filter_by_objectid(self):
+    def test_generic_reference_is_not_tracked_in_parent_doc(self):
+        """Ensure that modifications of related documents (through generic reference) don't influence
+        the owner changed fields (#1934)
+        """
+        class Doc1(Document):
+            name = StringField()
+
+        class Doc2(Document):
+            ref = GenericReferenceField()
+            refs = ListField(GenericReferenceField())
+
+        Doc1.drop_collection()
+        Doc2.drop_collection()
+
+        doc1 = Doc1(name='garbage1').save()
+        doc11 = Doc1(name='garbage11').save()
+        doc2 = Doc2(ref=doc1, refs=[doc11]).save()
+
+        doc2.ref.name = 'garbage2'
+        self.assertEqual(doc2._get_changed_fields(), [])
+
+        doc2.refs[0].name = 'garbage3'
+        self.assertEqual(doc2._get_changed_fields(), [])
+        self.assertEqual(doc2._delta(), ({}, {}))
+
+    def test_generic_reference_field(self):
         """Ensure we can search for a specific generic reference by
         providing its DBRef.
         """
@@ -4348,6 +4373,7 @@ class TestEmbeddedDocumentField(MongoDBTestCase):
             class MyFailingdoc2(Document):
                 emb = EmbeddedDocumentField('MyDoc')
 
+
 class CachedReferenceFieldTest(MongoDBTestCase):
 
     def test_cached_reference_field_get_and_save(self):