Fixed querying ReferenceField custom_id (MongoEngine#317)

Author: rozza

docs/changelog.rst

@@ -4,6 +4,7 @@ Changelog
 
 Changes in 0.8.0
 ================
+- Fixed querying ReferenceField custom_id (#317)
 - Fixed pickle issues with collections (#316)
 - Added `get_next_value` preview for SequenceFields (#319)
 - Added no_sub_classes context manager and queryset helper (#312)

mongoengine/fields.py

@@ -853,8 +853,6 @@ def to_mongo(self, document):
             if not self.dbref:
                 return document.id
             return document
-        elif not self.dbref and isinstance(document, basestring):
-            return ObjectId(document)
 
         id_field_name = self.document_type._meta['id_field']
         id_field = self.document_type._fields[id_field_name]