Mongo aggregate $slice to specific value instead of array

Question

I have a document in my collection:

  {
    "_id" : ObjectId("5b8aaaebf57de10e080c9151"),
    "user_email" : "[email protected]",
    "platforms_budget" : [ 
        {
            "_id" : ObjectId("5b8aaaebf57de10e080c9154"),
            "platform_id" : "f_01",
            "platform_name" : "Facebook"
        }, 
        {
            "_id" : ObjectId("5b8aaaebf57de10e080c9153"),
            "platform_id" : "i_01",
            "platform_name" : "Instagram"
        }, 
        {
            "_id" : ObjectId("5b8aaaebf57de10e080c9152"),
            "platform_id" : "f_02",
            "platform_name" : "Facebook_Adds"

        }
    ],
    "__v" : 0
    }

I want to find specific user by "user_email" and get only the length of the relevant "platform_budget" array.

My function is like this:

var BudgetSchema = require('../models/Budget');

  router.post('/temp', async function (req, res) {
  var length = await BudgetSchema.aggregate(
    [{ $match: { user_email: "[email protected]" } },
    { $project: { "count": { $size: '$platforms_budget' } } }])

  console.log(length);
})

The output that I get is:

[ { _id: 5b8aaaebf57de10e080c9151, count: 3 } ]

But I want to get only the size as a number, like this: 3.

The output that I want to get is:

I understand that I can use the $slice operator, but I can't figure how to use it in the right way. Any ideas? Thanks.

Ashh · Accepted Answer · 2018-09-02 17:10:30Z

1

Your aggregation should be like this

BudgetSchema.aggregate([
  { "$match": { "user_email": "[email protected]" } },
  { "$project": { "count": { "$size": "$platforms_budget" }}}
])

Output

[ { _id: "5b8aaaebf57de10e080c9151", count: 3 } ]

If you want it as an array value

const budget = (await BudgetSchema.aggregate([
  { "$match": { "user_email": "[email protected]" } },
  { "$project": { "count": { "$size": "$platforms_budget" }}}
])).map(({ count }) => count)

Output

[3]

edited Sep 2, 2018 at 17:10

answered Sep 2, 2018 at 16:54

Ashh

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4 Comments

gusya59 Over a year ago

Yes, thank you. I want to cut the output and accept only "3" or extract it from the responce in any other way.

gusya59 Over a year ago

I updated my question. I hope that is more clear now.

Ashh Over a year ago

your output is not a valid json... MongoDB itself won't return anything other than a BSON Document, and a simple "string or integer" is NOT a BSON Document.

gusya59 Over a year ago

Yes! Thank you very much for your help.

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