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  1. Core Server
  2. SERVER-36112

disparate behavior of $slice and $elemMatch projection operators

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    • Type: Icon: Bug Bug
    • Resolution: Duplicate
    • Priority: Icon: Minor - P4 Minor - P4
    • None
    • Affects Version/s: 3.6.6, 4.0.0
    • Component/s: Querying
    • Labels:
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    • ALL
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       >db.tests.find().pretty()
{
        "_id" : ObjectId("5b487b56f00b9013cff5a4dd"),
        "txid" : 0,
        "input" : [
                {
                        "_id" : ObjectId("5b487b56f00b9013cff5a4de"),
                        "pid" : 0,
                        "value" : 42
                }
        ],
        "output" : [
                {
                        "_id" : ObjectId("5b487b56f00b9013cff5a4e2"),
                        "value" : 123,
                        "n" : 0
                },
                {
                        "_id" : ObjectId("5b487b56f00b9013cff5a4e1"),
                        "value" : 345,
                        "n" : 1
                },
                {
                        "_id" : ObjectId("5b487b56f00b9013cff5a4e0"),
                        "value" : 567,
                        "n" : 2
                },
                {
                        "_id" : ObjectId("5b487b56f00b9013cff5a4df"),
                        "value" : 789,
                        "n" : 3
                }
        ],
        "__v" : 0
}
>var cond = {txid: 0}
>var update = { $set: { ['output.0.value']: 777 } };
>var opts = { projection: { _id: 0, output: { $slice: [0,1] } }, returnNewDocument: true };
>db.tests.findOneAndUpdate(cond, update, opts);
{
        "txid" : 0,
        "input" : [
                {
                        "_id" : ObjectId("5b487b56f00b9013cff5a4de"),
                        "pid" : 0,
                        "value" : 42
                }
        ],
        "output" : [
                {
                        "_id" : ObjectId("5b487b56f00b9013cff5a4e2"),
                        "value" : 777,
                        "n" : 0
                }
        ],
        "__v" : 0
}
>var update = { $set: { ['output.0.value']: 123 } };
>var opts = { projection: { _id: 0, output: { $elemMatch: { n: 0 } } }, returnNewDocument: true };
>db.tests.findOneAndUpdate(cond, update, opts);
{
        "output" : [
                {
                        "_id" : ObjectId("5b487b56f00b9013cff5a4e2"),
                        "value" : 123,
                        "n" : 0
                }
        ]
}
>
      Show
       >db.tests.find().pretty() { "_id" : ObjectId( "5b487b56f00b9013cff5a4dd" ), "txid" : 0, "input" : [ { "_id" : ObjectId( "5b487b56f00b9013cff5a4de" ), "pid" : 0, "value" : 42 } ], "output" : [ { "_id" : ObjectId( "5b487b56f00b9013cff5a4e2" ), "value" : 123, "n" : 0 }, { "_id" : ObjectId( "5b487b56f00b9013cff5a4e1" ), "value" : 345, "n" : 1 }, { "_id" : ObjectId( "5b487b56f00b9013cff5a4e0" ), "value" : 567, "n" : 2 }, { "_id" : ObjectId( "5b487b56f00b9013cff5a4df" ), "value" : 789, "n" : 3 } ], "__v" : 0 } > var cond = {txid: 0} > var update = { $set: { [ 'output.0.value' ]: 777 } }; > var opts = { projection: { _id: 0, output: { $slice: [0,1] } }, returnNewDocument: true }; >db.tests.findOneAndUpdate(cond, update, opts); { "txid" : 0, "input" : [ { "_id" : ObjectId( "5b487b56f00b9013cff5a4de" ), "pid" : 0, "value" : 42 } ], "output" : [ { "_id" : ObjectId( "5b487b56f00b9013cff5a4e2" ), "value" : 777, "n" : 0 } ], "__v" : 0 } > var update = { $set: { [ 'output.0.value' ]: 123 } }; > var opts = { projection: { _id: 0, output: { $elemMatch: { n: 0 } } }, returnNewDocument: true }; >db.tests.findOneAndUpdate(cond, update, opts); { "output" : [ { "_id" : ObjectId( "5b487b56f00b9013cff5a4e2" ), "value" : 123, "n" : 0 } ] } >

      When using the $slice projection operator on the output field below, the input and txid fields are included in the projection. When using the $elemMatch projection operator on the output field, the txid and input fields are not included in the projection. I believe that the correct behavior is exhibited by the $elemMatch operator, and should be the same with $slice.

            Assignee:
            nick.brewer Nick Brewer
            Reporter:
            lineus@gmail.com Kevin Huff
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              Created:
              Updated:
              Resolved: