MongoEngine/mongoengine — pull request #254 by Gerrrr (2013-03-16 16:07:37+00:00)
Title: ReferenceField query chaining bug fixed.

The problem was caused by absence of __deepcopy__ implementation in QuerySet. We need it in visitors' _query_conjunction when using ReferenceField in query as it transforms to QuerySet which can't be deepcopied by default.