addToSet
db.test.find()
{ “_id” : ObjectId(“51dfb4f18591f90cbb889429”), “A” : 1, “B” : 2 }**{ “_id” : ObjectId(“51dfb4f58591f90cbb88942a”), “A” : 3, “B” : 2 } { “_id” : ObjectId(“51dfb4f68591f90cbb88942b”), “A” : 3, “B” : 4 } { “_id” : ObjectId(“51dfb4f88591f90cbb88942c”), “A” : 5, “B” : 4 } { “_id” : ObjectId(“51dfb4fa8591f90cbb88942d”), “A” : 7, “B” : 4 } db.test.aggregate({“$group”:{“_id”:”$A”,”cnt”:{“$sum”:1},”B”:{“$addToSet”:”$B”}}}) { “result” : [ { “_id” : 7, “cnt” : 1, “B” : [ 4 ] }, { “_id” : 5, “cnt” : 1, “B” : [ 4 ] }, { “_id” : 3, “cnt” : 2, “B” : [ 4, 2 ] }, { “_id” : 1, “cnt” : 1, “B” : [ 2 ] } ], “ok” : 1 }