mongodb - Return array of elements from multiple arrays -

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i got collection of companies looks this. want merge other documents deals. need this:

{      "_id" : objectid("561637942d25a7644cae993e"),      "locations" : [         {             "deals" : [                 {                     "name" : "1",                      "_id" : objectid("561637942d25a7644cae9940")                 },                  {                     "name" : "2",                      "_id" : objectid("562f868ce73962c626a16b15")                 }             ]         }     ],      "deals" : [         {             "name" : "3",              "_id" : objectid("562f86ebe73962c626a16b17")         }     ] } {      "_id" : objectid("561637942d25a7644cae993e"),      "locations" : [         {             "deals" : [                 {                     "name" : "4",                      "_id" : objectid("561637942d25a7644cae9940")                 }             ]         }     ],      "deals" : [] }

to this:

{     "deals": [{         "name" : "1",          "_id" : objectid("561637942d25a7644cae9940")     },{         "name" : "2",          "_id" : objectid("562f868ce73962c626a16b15")     },{         "name" : "3",          "_id" : objectid("562f86ebe73962c626a16b17")     },{         "name" : "4",          "_id" : objectid("561637942d25a7644cae9949")     }] }

but have failed this. seems if want deals grouped 1 array should not use unwind since create more documents because need group once. attempt not work @ all.

{                 "$project": {                     "_id": 1,                     "locations": 1,                     "deals": 1                 }             }, {                 "$unwind": "$locations"             }, {                 "$unwind": "$locations.deals"             }, {                 "$unwind": "$deals"             }, {                 "$group": {                     "_id": null,                     "deals": {                          "$addtoset": "$locations.deals",                         "$addtoset": "$deals"                     }                 }             }

you should first use filter documents reduce size of documents process in pipeline using $match operator. need $unwind "locations" array after use $project operator reshape documents. $cond operator used return single element array [false] if deals field empty array or deals value because $unwinding empty array throw exception. of course $setunion operator return array of element appear in locations.deals array or deals array. use $setdifference operator filter out false element merged array. need $unwind stage deconstruct deals array. there can $group documents.

db.collection.aggregate([     { "$match": { "locations.0": { "$exists": true } } },     { "$unwind": "$locations" },      { "$project": {          "deals": {              "$setdifference": [                 { "$setunion": [                        { "$cond": [                          { "$eq" : [ { "$size": "$deals" }, 0 ] },                           [false],                           "$deals"                      ]},                      "$locations.deals"                 ]},                  [false]             ]          }     }},     { "$unwind": "$deals" },      { "$group": {          "_id": null,          "deals": { "$addtoset": "$deals" }     }} ])

which returns:

{         "_id" : null,         "deals" : [                 {                         "name" : "1",                         "_id" : objectid("561637942d25a7644cae9940")                 },                 {                         "name" : "2",                         "_id" : objectid("562f868ce73962c626a16b15")                 },                 {                         "name" : "3",                         "_id" : objectid("562f86ebe73962c626a16b17")                 },                 {                         "name" : "4",                         "_id" : objectid("561637942d25a7644cae9940")                 }         ] }

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