Mongodb Aggregation framework group and sort

Question

I have the following document structure...

{
  "id":"documentID"
  "sessionId":"sometext"
  "msg":"sometext"
  "time":"date"
}

sessionId can exist in many documents

I want to aggregate the documents by sessionId, the result for each session should contain the set of messages related to the session sorted by time.

Using the MongoDB aggregation framework how can I achieve that?

I have tried to sort first and then group but the messages in each session wasn't sorted for some reason:

{ $sort: { "time": 1 } },
{ "$group" : { 
    "_id" : "$sessionId", 
    "msgs" : { "$addToSet" : "$msg" }
} }

any suggestions? your answer is highly appreciated.

Thank you .. and if I want to get with each msg the time also in the result? — assaf_miz84
– assaf_miz84, Commented Aug 14, 2013 at 22:07

Mason · Accepted Answer · 2013-08-16 16:38:42Z

6

You can do this:

db.collection.aggregate( 
    {$sort:{"time":1}},
    { $group:
        { _id: "$sessionId",
        messages: { "$push": {message: "$msg", time: "$time"} }
        }
    } 
)

This will sort the collection based on time then group by session id. Each session ID group will have an array of sub-documents which contain the message and time of the message. By sorting then pushing the messages will be ordered by time in your messages array.

edited Aug 16, 2013 at 16:38

answered Aug 15, 2013 at 2:05

Mason

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