Fix dynamic embedded document updates

In order to allow setting new fields that did not previouly exist on dynamic embedded documents, the `lookup_member` functions on `EmbeddedDocumentField` and `GenericEmbeddedDocumentField` return dynamic fields as appropriate. This enables operations like `A.objects(...).update(foo__newfield="bar")`.

Resolves MongoEngine#2486

Author: mcsimps2

docs/changelog.rst

@@ -24,6 +24,7 @@ Development
 - BugFix - Calling .clear on a ListField wasn't being marked as changed (and flushed to db upon .save()) #2858
 - Improve error message in case a document assigned to a ReferenceField wasn't saved yet #1955
 - BugFix - Take `where()` into account when using `.modify()`, as in MyDocument.objects().where("this[field] >= this[otherfield]").modify(field='new') #2044
+- BugFix - Unable to add new fields during `QuerySet.update` on `DynamicEmbeddedDocument` fields #2486
 
 Changes in 0.29.0
 =================

mongoengine/fields.py

@@ -777,6 +777,12 @@ def lookup_member(self, member_name):
             if field:
                 return field
 
+        # DynamicEmbeddedDocuments should always return a field except for positional operators
+        if any(
+            doc_type._dynamic for doc_type in doc_and_subclasses
+        ) and member_name not in ("$", "S"):
+            return DynamicField(db_field=member_name)
+
     def prepare_query_value(self, op, value):
         if value is not None and not isinstance(value, self.document_type):
             # Short circuit for special operators, returning them as is
@@ -837,6 +843,12 @@ def lookup_member(self, member_name):
                 if field:
                     return field
 
+        # DynamicEmbeddedDocuments should always return a field except for positional operators
+        if any(
+            document_choice._dynamic for document_choice in document_choices
+        ) and member_name not in ("$", "S"):
+            return DynamicField(db_field=member_name)
+
     def to_mongo(self, document, use_db_field=True, fields=None):
         if document is None:
             return None

tests/fields/test_embedded_document_field.py

@@ -6,6 +6,7 @@
 
 from mongoengine import (
     Document,
+    DynamicEmbeddedDocument,
     EmbeddedDocument,
     EmbeddedDocumentField,
     EmbeddedDocumentListField,
@@ -224,6 +225,34 @@ class Record(Document):
 
         assert Record.objects(posts__title="foo").count() == 2
 
+    def test_update_dynamic_embedded_document_with_new_fields(self):
+        class Wheel(DynamicEmbeddedDocument):
+            position = StringField()
+
+        class Car(Document):
+            wheels = EmbeddedDocumentListField(Wheel)
+
+        car = Car(
+            wheels=[
+                Wheel(position="front-passenger"),
+                Wheel(position="rear-passenger"),
+                Wheel(position="front-driver"),
+                Wheel(position="rear-driver"),
+            ]
+        ).save()
+
+        Car.objects(wheels__position="front-driver").update(
+            set__wheels__S__damaged=True
+        )
+        car.reload()
+
+        for wheel in car.wheels:
+            if wheel.position == "front-driver":
+                assert wheel.damaged
+            else:
+                with pytest.raises(AttributeError):
+                    wheel.damaged
+
 
 class TestGenericEmbeddedDocumentField(MongoDBTestCase):
     def test_generic_embedded_document(self):
@@ -455,3 +484,17 @@ class Person(Document):
 
         copied_map_emb_doc = deepcopy(doc.wallet_map)
         assert copied_map_emb_doc["test"]._instance is None
+
+    def test_update_dynamic_embedded_document_with_new_fields(self):
+        class Laptop(DynamicEmbeddedDocument):
+            operating_system = StringField()
+
+        class Backpack(Document):
+            content = GenericEmbeddedDocumentField(choices=[Laptop])
+
+        backpack = Backpack(content=Laptop(operating_system="Windows")).save()
+
+        Backpack.objects.update(set__content__manufacturer="Acer")
+        backpack.reload()
+
+        assert backpack.content.manufacturer == "Acer"