Fixed querying string versions of ObjectIds issue with ReferenceField (#307)

mongoengine/fields.py

@@ -854,7 +854,7 @@ class ReferenceField(BaseField):
                 return document.id
             return document
         elif not self.dbref and isinstance(document, basestring):
-            return document
+            return ObjectId(document)
 
         id_field_name = self.document_type._meta['id_field']
         id_field = self.document_type._fields[id_field_name]

mongoengine/queryset/queryset.py

@@ -544,8 +544,9 @@ class QuerySet(object):
         return c
 
     def select_related(self, max_depth=1):
-        """Handles dereferencing of :class:`~bson.dbref.DBRef` objects to
-        a maximum depth in order to cut down the number queries to mongodb.
+        """Handles dereferencing of :class:`~bson.dbref.DBRef` objects or
+        :class:`~bson.object_id.ObjectId` a maximum depth in order to cut down
+        the number queries to mongodb.
 
         .. versionadded:: 0.5
         """