Array is reordered when using $lookup

I have this aggregation:

db.getCollection("users").aggregate([
  {
    "$match": {
      "_id": "5a708a38e6a4078bd49f01d5"
    }
  },
  {
    "$lookup": {
      "from": "user-locations",
      "localField": "locations",
      "as": "locations",
      "foreignField": "_id"
    }
  }
])

It works well, but there is one small thing that I don't understand and I can't fix. In the query output, the locations array is reordered by ObjectId and I really need to keep the original order of data.

Here is how the locations array from the users collection looks like

'locations' : [
     ObjectId("5b55e9820b720a1a7cd19633"), 
     ObjectId("5a708a38e6a4078bd49ef13f")
],

And here is the result after the aggregation:

'locations' : [
     {
        '_id' : ObjectId("5a708a38e6a4078bd49ef13f"),
        'name': 'Location 2'
     },
     {
        '_id' : ObjectId("5b55e9820b720a1a7cd19633"),
        'name': 'Location 1'
     }
],

What am I missing here? I really have no idea how to proceed with this issue. Could you give me a push?

Solution

$lookup does not guarantee order of result documents, you can try a approach to manage natural order of document,

$unwind deconstruct locations array and add auto index number will start from 0,
$lookup with locations
$set to select first element from locations
$sort by index field in ascending order
$group by _id and reconstruct locations array

db.users.aggregate([
  { $match: { _id: "5a708a38e6a4078bd49f01d5" } },
  {
    $unwind: {
      path: "$locations",
      includeArrayIndex: "index"
    }
  },
  {
    $lookup: {
      from: "user-locations",
      localField: "locations",
      foreignField: "_id",
      as: "locations"
    }
  },
  { $set: { locations: { $arrayElemAt: ["$locations", 0] } } },
  { $sort: { index: 1 } },
  {
    $group: {
      _id: "$_id",
      locations: { $push: "$locations" }
    }
  }
])

Playground