Info
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.
Top User Comments
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
Steps to Reproduce
>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
}
]
}
>
