class B(DynamicEmbeddedDocument):
   foo = StringField()

class A(Document):
  b = EmbeddedDocumentField(B)

# This works
A.objects(...).update(set__b__foo="123")

# This does not work
A.objects(...).update(set__b__bar="123")
# mongoengine.errors.InvalidQueryError: Cannot resolve field "bar"

class EmbeddedDocumentField(BaseField):
      def lookup_member(self, member_name):
        doc_and_subclasses = [self.document_type] + self.document_type.__subclasses__()
        for doc_type in doc_and_subclasses:
            field = doc_type._fields.get(member_name)
            if field:
                return field
        # NEW CODE
        if member_name not in ("S", "$") and any(issubclass(doc_type, DynamicEmbeddedDocument) for doc_type in doc_and_subclasses):
           return DynamicField(db_field=member_name)

A.objects(b__foo="abc").modify(set__list_of_b__S__bar="123")