User: WilliamZhu
Date: 12-10-16
Time: 下午8:11
The +Document+ class is the Parent Class of All MongoDB model,
this means any MongoDB model class should extends this class.
User: WilliamZhu
Date: 12-10-17
Time: 下午3:21
The +MongoMongo+ class is the wrapper of MongoDB driver in order to
get +DB+ Object.So we can use +DB+ to get collection