You can't use DBRef in an aggregation pipeline. From the Aggregation Guide:
Aggregation Pipeline Limits
Aggregation operations with the aggregate command have the following limitations.
Type Restrictions The aggregation pipeline (page 7) cannot operate on values of the following types: Symbol, MinKey, MaxKey, DBRef, Code, and CodeWScope.
http://docs.mongodb.org/manual/MongoDB-aggregation-guide.pdf