有大神知道怎么通过mongoose的聚合来实现文章归档的功能么？ { "_id" : ObjectId(“51b6e603e3efef161b000003”), “title”: “文章1”， “created” : ISODate(“2017-12-7T08:55:31.957Z”) } { "_id" : ObjectId(“51b6e603e3efef161b000003”), “title”: “文章2”， “created” : ISODate(“2017-12-6T08:55:31.957Z”) } { "_id" : ObjectId(“51b6e603e3efef161b000003”), “title”: “文章3”， “created” : ISODate(“2017-11-11T08:55:31.957Z”) }

怎么通过聚合来达到这种效果 [ { year: 2017, monthlyusage: [ { month: 12, article: [ { “_id”: xxxxx, title: “文章1” }， { “_id”: xxxxx, title: “文章2” } ] }， { month: 11, article: [ { “_id”: xxxxx, title: “文章3” } ] } ] }， { year: 2016, xxxxxx } ]

这种聚合怎么写啊