Merge pull request #1913 from bagerard/1453_mapfield_list_reference_field

Fix complex combinations of DictField(ListField(ReferenceField))

mongoengine/base/document.py

@@ -2,9 +2,7 @@ import copy
 import numbers
 from functools import partial
 
-from bson import ObjectId, json_util
-from bson.dbref import DBRef
-from bson.son import SON
+from bson import DBRef, ObjectId, SON, json_util
 import pymongo
 import six