Customized function in $group operator of aggregation framework in MongoDB

Question

I want to use a customized $sum (lets call it $boolSum) function that returns the number of true element in an array. e.g.

group :{ _id: { a : '$a' b : '$b', c : '$c' }, d1: { $boolSum : '$d1'} d2: { $boolSum : '$d2'} }

But it seems that there is no way to define boolSum function. I tried to add a new record in system.js, but it doesn't work.

In MongoDB, is it possible to cutomize function in $group operator of aggregation framework?

Map/reduce and the aggregation framework work very differently. In the former, you can use all features of javascript and define your own methods. The latter relies on operators that are implemented internally in mongodb. However, M/R comes with a lot of overhead and, unlike the aggregation framework, wasn't really made for ad-hoc queries. — mnemosyn
– mnemosyn, Commented May 27, 2015 at 12:45

Sylvain Leroux · Accepted Answer · 2015-05-27 12:49:21Z

is it possible to cutomize function in $group operator of aggregation framework

No, not really...

I want to use a customized $sum (lets call it $boolSum) function that returns the number of true element in an array

... but you can combine $sum with other operators (like $cond in that particular case) to achieve the desired result:

{$sum: {$cond:["$someBooleanField",1,0]}}

or

{$sum: {$cond:[{$eq: ["$somePossiblyBooleanField",true]},1,0]}}

It is similar to my current solution but seems to be better. Unlike your solution I used the $project operator which uses $cond to convert bool to int. I will try yours and see if it is faster than mine.

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