sorting - MongoDB: Sort in combination with Aggregation group -


i have collection called transaction below documents,

/* 0 */ {     "_id" : objectid("5603fad216e90d53d6795131"),     "statusid" : "65c719e6727d",     "relatedwith" : "65c719e67267",     "status" : "a",     "userid" : "100",     "createdts" : isodate("2015-09-24t13:15:36.609z") }  /* 1 */ {     "_id" : objectid("5603fad216e90d53d6795134"),     "statusid" : "65c719e6727d",     "relatedwith" : "65c719e6726d",     "status" : "b",     "userid" : "100",     "createdts" : isodate("2015-09-24t13:14:31.609z") }  /* 2 */ {     "_id" : objectid("5603fad216e90d53d679512e"),     "statusid" : "65c719e6727d",     "relatedwith" : "65c719e6726d",     "status" : "c",     "userid" : "100",     "createdts" : isodate("2015-09-24t13:13:36.609z") }  /* 3 */ {     "_id" : objectid("5603fad216e90d53d6795132"),     "statusid" : "65c719e6727d",     "relatedwith" : "65c719e6726d",     "status" : "d",     "userid" : "100",     "createdts" : isodate("2015-09-24t13:16:36.609z") }

when run below aggregation query without $group,

db.transaction.aggregate([     {         "$match": {             "userid": "100",             "statusid": "65c719e6727d"         }     },     {         "$sort": {             "createdts": -1         }     } ])

i result in expected sorting order. i.e sort createdts in descending order (minimal result)

/* 0 */ {     "result" : [          {             "_id" : objectid("5603fad216e90d53d6795132"),             "createdts" : isodate("2015-09-24t13:16:36.609z")         },          {             "_id" : objectid("5603fad216e90d53d6795131"),             "createdts" : isodate("2015-09-24t13:15:36.609z")         },          {             "_id" : objectid("5603fad216e90d53d6795134"),             "createdts" : isodate("2015-09-24t13:14:31.609z")         },          {             "_id" : objectid("5603fad216e90d53d679512e"),             "createdts" : isodate("2015-09-24t13:13:36.609z")         }     ],     "ok" : 1 }

if apply below aggregation $group, resultant inversely sorted(i.e ascending sort)

db.transaction.aggregate([     {         "$match": {             "userid": "100",             "statusid": "65c719e6727d"         }     },     {         "$sort": {             "createdts": -1         }     },     {         $group: {                 "_id": {                     "statusid": "$statusid",                     "relatedwith": "$relatedwith",                     "status": "$status"                 },                 "status": {$first: "$status"},                  "statusid": {$first: "$statusid"},                 "relatedwith": {$first: "$relatedwith"},                 "createdts": {$first: "$createdts"}         }     } ]);

i result in inverse order i.e. ** sort createdts in ascending order**

/* 0 */ {     "result" : [          {             "_id" : objectid("5603fad216e90d53d679512e"),             "createdts" : isodate("2015-09-24t13:13:36.609z")         },          {             "_id" : objectid("5603fad216e90d53d6795134"),             "createdts" : isodate("2015-09-24t13:14:31.609z")         },          {             "_id" : objectid("5603fad216e90d53d6795131"),             "createdts" : isodate("2015-09-24t13:15:36.609z")         },          {             "_id" : objectid("5603fad216e90d53d6795132"),             "createdts" : isodate("2015-09-24t13:16:36.609z")         }     ],     "ok" : 1 }

where wrong ?

the $group stage doesn't insure ordering of results. see here first paragraph. if want results sorted after $group, need add $sort after $group stage.

in case, should move $sort after $group , before ask question : no, $sort won't able use index after $group before $group :-).

the internal algorithm of $group seems keep sort of ordering (reversed apparently), not count on , add $sort.


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