OPERATIONAL DEFECT DATABASE
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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.
lineus@gmail.com commented on Mon, 16 Jul 2018 22:57:33 +0000: Thanks for the education I can't believe I missed that in the docs! nick.brewer commented on Mon, 16 Jul 2018 21:18:17 +0000: lineus@gmail.com, I believe this is a duplicate of an older ticket (SERVER-3378), which contains an explanation of the expected behavior. Our documentation has some additional information related the use of projection against arrays - as it mentions, $elemMatch is the appropriate tool for this use case. Regards, Nick
>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 } ] } >
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