你用哪个mongodb的模块啊, mongoskin还是mongoose之类的 https://github.com/kissjs/node-mongoskin#groupkeys-condition-initial-reduce-command-callback

@sinple 没用过, google到的: http://jeroenbourgois.be/mongodb-group-by-with-mongoose/ http://stackoverflow.com/questions/6852588/group-by-in-mongoose

@sinple 看下stackoverflow的那个,第一个回答.还有这个: http://www.mongodb.org/display/DOCS/Aggregation#Aggregation-Group

key是可以key: { a:true, b:true },