MongoDB slice array and retrieve only that array field in projection

Question

I have a mongodb schema where documents are as per below structure ,

{"_id":"5e4aaadefrbff2150c42ed5b",
"data":[{"time":"15:24:31","amount":"34"},{"time":"15:24:33","amount":"38"},,{"time":"15:24:35","amount":"95"}],
"code":"S04",
"rollnumber":4,
"intime":"2020-02-16T12:24:21.921Z"}

And in my node.js code i am fetching data in this way to find the last array value,

mycollection.find({ code: "S04", rollnumber: 4 }, { "_id": 0, code: 0, rollnumber: 0, intime: 0, data: { $slice: -1 } }, (err, result) => {
            if (err) { console.log('err', err); }
            console.log(result)
            res.json(result);
        });

The problem is, i want only array result hence put all other keys:0 . It doesn't work when i put only data:1, data: { $slice: -1 }

It works when in put data:1 without slice operator. In production the list is going to be a big so i can not put all the keys with :0 in query. Please suggest how to put $slice with :1 to display only filtered array.

whoami - fakeFaceTrueSoul · Accepted Answer · 2020-02-18 15:43:09Z

2

It doesn't work like that, if you include _id it should work, but it you just need data then alternatively you can try this :

db.collection.aggregate([
              {$match : {code: "S04", rollnumber: 4}},
              {$project : { _id :0, data : {$slice : [ "$data", -1 ]}}}
])

Test : MongoDB-Playground

edited Feb 18, 2020 at 15:43

answered Feb 18, 2020 at 15:38

whoami - fakeFaceTrueSoul

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