Mongo aggregate $slice to object instead of array

Question

I have upgraded Mongo to 3.2 and am delighted that aggregating using $slice works. However, my problem is that I want to assign the value to an object not an array. I cannot find how to do this.

My script applied:

db.std_sourceBusinessData.aggregate(
  { $match : {objectType: "Account Balances"}},
  { $project: {_id: 1,entity_id: 1,accountBalances: 1}},
  { $unwind: "$accountBalances" },
  { $match: {"accountBalances": "Sales"}},
  {$project: {
      _id :1, 
      "Value" : {$slice: ["$accountBalances",1,1]},
      "key" : {$literal: "sales"},
      "company": "$entity_id"
  }}
)

Comes back with:

{
"_id" : ObjectId("566f3da3d58419e8b0fc76c7"),
"Value" : [ 
    "5428.64"
],
"key" : "sales"
}

Notice that Value is an array. What I want is:

{
"_id" : ObjectId("566f3da3d58419e8b0fc76c7"),
"Value" : "5428.64",
"key" : "sales"
}

Thanks, Matt

add an unwind on Value to aggregation - {$unwind: '$Value'}

Sarath Nair
– Sarath Nair

2015-12-15 04:17:03 +00:00
Commented Dec 15, 2015 at 4:17 — Sarath Nair
– Sarath Nair, Commented Dec 15, 2015 at 4:17

JohnnyHK · Accepted Answer · 2015-12-15 04:52:49Z

4

You can use $arrayElemAt instead of $slice to directly get a single array element.

Modify your final $project stage to be:

{$project: {
    _id: 1, 
    "Value": {$arrayElemAt: ["$accountBalances", 1]}, 
    "key": {$literal: "sales"},
    "company": "$entity_id"
}}

edited Dec 15, 2015 at 4:52

answered Dec 15, 2015 at 4:47

JohnnyHK

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3 Comments

Matt Lightbourn Over a year ago

That is great Johnny, awesome. That works, one small problem I have just noticed, the "company": "$entity_id" isn't showing. It is in the origin document, is there something I'm missing in here to retrieve values from the origin document directly? Thanks

JohnnyHK Over a year ago

You're including entity_id in your initial $project so that should work fine. Comment out the later pipeline stages and see where it's getting lost. If you're still stuck, post a new question as it's unrelated to this one.

Matt Lightbourn Over a year ago

cool, thanks, I worked out that the first one has ID instead of id, cases sensitivity issue, thanks, Matt

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