mongodb - Sorting aggregation addToSet result

Question

Is there a way to get result of $addToSet as sorted array ?

I tried to expand the pipeline and $unwind the array, sort it and group it again , but still the result isn't sorted.

The arrays are pretty big and i try to avoid sort them in the the application.

Document Example :

    {
      "_id" : ObjectId("52a84825cc8391ab188b4567"),
      "id" : 129624
      "message" : "Sample",
      "date" : "12-09-2013,17:34:34",
      "dt" : ISODate("2013-12-09T17:34:34.000Z"),

    }

Query :

    db.uEvents.aggregate(
    [
      {$match : {dt : {$gte : new Date(2014,01,01) , $lt : new Date(2015,01,17)}}}
      ,{$sort : {dt : 1}}
      , {$group : {
        _id : {
                id : "$id"
                , year : {'$year' : "$dt"}
                , month : {'$month' : "$dt"}
                , day : {'$dayOfMonth' : "$dt"}
            }
        ,dt : {$addToSet : "$dt"}

      }}    
    ]

    );

Answer 1

Yes it is possible, but approach it differently. I'm just provide my own data for this, but you'll get the concept.

My Sample:

{ "array" : [  2,  4,  3,  5,  2,  6,  8,  1,  2,  1,  3,  5,  9,  5 ] }

I'm going to "semi-quote" the CTO on this and state that Sets are considered to be unordered.

There is an actual JIRA, Google groups statement that goes something like that. So let's take it from "Elliot" and accept that this will be the case.

So if you want an ordered result, you have to massage that way with stages like this

db.collection.aggregate([

    // Initial unwind
    {"$unwind": "$array"},

    // Do your $addToSet part
    {"$group": {"_id": null, "array": {"$addToSet": "$array" }}},

    // Unwind it again
    {"$unwind": "$array"},

    // Sort how you want to
    {"$sort": { "array": 1} },

    // Use $push for a regular array
    {"$group": { "_id": null, "array": {"$push": "$array" }}}

])

And then do whatever. But now your array is sorted.