How to use a variable in an aggregate query in mongoose/node.js

Question

I've got a very basic Mongoose aggregate query

Transaction.aggregate().match({
    timestamp : {
        $gt : time_lower_bound,
        $lt : time_upper_bound
    }
}).group({
    _id : '$currencyFrom',
    count : {
        $sum : 1
    }
}).sort({
    count : -1
})

I wanted to make the _id dynamic. But i tried

var field = job.data.field;
Transaction.aggregate().match({
    timestamp : {
        $gt : time_lower_bound,
        $lt : time_upper_bound
    }
}).group({
    _id : '$' . field,
    count : {
        $sum : 1
    }
}).sort({
    count : -1
})

and it doesn't return the correct data, it returns

[ { _id: null, count: 473 } ]

so i'm missing something obvious here.

adeneo · Accepted Answer · 2015-02-24 17:58:26Z

2

Javascript uses plus signs for concatenation, not periods.

Assuming field is a string like currencyFrom, replace

_id : '$' . field,

with

_id : '$' + field,

answered Feb 24, 2015 at 17:58

adeneo

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