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  1. Core Server
  2. SERVER-32947

Does mongodb $lookup operator ruins default array order?

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    • Type: Icon: Bug Bug
    • Resolution: Done
    • Priority: Icon: Major - P3 Major - P3
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    • Affects Version/s: 3.4.10
    • Component/s: Aggregation Framework
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      1. Get some test docs with array field, filled with ObjectId elements.
      2. Run this query.

      db.getCollection('trades').aggregate([
{  '$match': {
        _id: ObjectId('5a546428f228c3156579fe00')
    } 
}, 
{ 
    '$lookup': { 
        from: 'images',
        localField: 'images',
        foreignField: '_id',
        as: 'images'
    }
}
]);

      3. Default order is ruined.

      Show
      1. Get some test docs with array field, filled with ObjectId elements. 2. Run this query. db.getCollection( 'trades' ).aggregate([ { '$match' : { _id: ObjectId( '5a546428f228c3156579fe00' ) } }, { '$lookup' : { from: 'images' , localField: 'images' , foreignField: '_id' , as: 'images' } } ]); 3. Default order is ruined.
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      I faced with some strange things when I was working with mongoose. I got mongodb collection with some docs:

      {
"_id" : ObjectId("5a0bf58c9616e5016426d5ef"),
"categoryId" : 101,
"name" : "Some name of something",
"price" : 50000,
"images" : [ 
    ObjectId("5a0bf5349616e5016426d5e9"),
    ObjectId("5a0bf6ac9616e5016426d5f7")
],
}

      When I join images in "images" field by ObjectId, $lookup operator jumble up default order of array. Why is this happening? When I use third-party ODM (Mongoose) to join images default order of images array is preserved.

      db.getCollection('trades').aggregate([
{  '$match': {
        _id: ObjectId('5a546428f228c3156579fe00')
    } 
}, 
{ 
    '$lookup': { 
        from: 'images',
        localField: 'images',
        foreignField: '_id',
        as: 'images'
    }
}
]);

            Assignee:
            mark.agarunov Mark Agarunov (Inactive)
            Reporter:
            penpaul Pavel
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              Created:
              Updated:
              Resolved: