node.js - MongoDB: Concatenate Multiple Arrays -

i have 3 arrays of objectids want concatenate single array, , sort creation date. $setunion precisely want, i'd try without using it.

schema of object want sort:

var chirpschema = new mongoose.schema({     interactions: {        _liked      : ["55035390d3e910505be02ce2"] // [{ type: $oid, ref: "interaction" }]     , _shared     : ["507f191e810c19729de860ea", "507f191e810c19729de860ea"] //  [{ type: $oid, ref: "interaction" }]     , _viewed     : ["507f1f77bcf86cd799439011"] //  [{ type: $oid, ref: "interaction" }]   } }); 

desired result: concatenate _liked, _shared, , _viewed single array, , sort them creation date using aggregate pipeline. see below

["507f1f77bcf86cd799439011", "507f191e810c19729de860ea", "507f191e810c19729de860ea", "55035390d3e910505be02ce2"] 

i know i'm suppose use $push, $each, $group, , $unwind in combination or other, i'm having trouble piecing documenation make happen.

update: query

model_user.aggregate([       { $match    : { '_id' : { $in : following } } }     , { $project  : { 'interactions' : 1 } }     , { $project  : {           "combined": { $setunion : [              "$interactions._liked"           , "$interactions._shared"           , "$interactions._viewed"         ]}       }} ]) .exec(function (err, data) {   if (err) return next(err);   next(data); // combined returning null }) 

if object _id values "unique" $setunion best option. of course not "ordered" in way works "set", , not guarantee order. can unwind , $sort.

[     { "$project": {         "combined": { "$setunion": [              { "$ifnull": [ "$interactions._liked", [] ] },             { "$ifnull": [ "$interactions._shared", [] ] },             { "$ifnull", [ "$interactions._viewed", [] ] }         ]}     }},     { "$unwind": "$combined" },     { "$sort": { "combined": 1 } },     { "$group": {         "_id": "$_id",         "combined": { "$push": "$combined" }     }} ] 

of course again since "set" of distinct values can old way instead $addtoset, after processing $unwind on each array:

[     { "$unwind": "$interactions._liked" },     { "$unwind": "$interactions._shared" },     { "$unwind": "$interactions._viewed" },     { "$project": {         "interactions": 1,         "type": { "$const": [ "liked", "shared", "viewed" ] }     }}     { "$unwind": "$type" },     { "$group": {         "_id": "$_id",         "combined": {             "$addtoset": {                 "$cond": [                    { "$eq": [ "$type", "liked" ] },                    "$interactions._liked",                    { "$cond": [                        { "$eq": [ "$type", "shared" ] },                        "$interactions._shared",                        "$interactions._viewed"                    ]}                 ]             }         }     }},     { "$unwind": "$combined" },     { "$sort": { "combined": 1 } },     { "$group": {         "_id": "$_id",         "combined": { "$push": "$combined" }     }} ] 

but still same thing applies ordering.

future releases have ability concatenate arrays without reducing "set":

[     { "$project": {         "combined": { "$concatarrays": [              "$interactions._liked",             "$interactions._shared",             "$interactions._viewed"         ]}     }},     { "$unwind": "$combined" },     { "$sort": { "combined": 1 } },     { "$group": {         "_id": "$_id",         "combined": { "$push": "$combined" }     }} ] 

but still there no way re-order results without procesing $unwind , $sort.

you might therefore consider unless need grouped across multiple documents, basic "contenate , sort" operation best handled in client code. mongodb has no way "in place" on array @ present, per document in client code best bet.

but if need grouping on multiple documents, sort of approaches shown here you.

also note "creation" here means creation of objectid value , not other properties referenced objects. if need those, perform populate on id values after aggregation or query instead, , of course sort in client code.