multi sum/count on mongodb (sum gender and total all results)

Question

i have this document:

{gender:"male", ...},
{gender:"male", ...},
{gender:"female", ...},
{gender:"female", ...},
{gender:"female", ...},

So, i need retrive like

{
total:5,
male:2,
female:3
}

my actual query(no work):

db.collection.aggregate([
{
    $match:{...}
},
{
    $group:{
        _id:"$_id",
        gender:{
            $push:"$gender"
        },
        total:{
            $sum:1
        }
    }
},
{
    $unwind:"$gender"
},
{
    $group:{
        _id:"$gender",
        name:{
            $addToSet:"$all"
        },
        "count":{
            $sum:1
        }
    }
}
])

how i can retrive counter of gender and total? thanks

Answer 1

Something like that will do the trick:

db.collection.aggregate([
  {$project: {
    male: {$cond: [{$eq: ["$gender", "male"]}, 1, 0]},
    female: {$cond: [{$eq: ["$gender", "female"]}, 1, 0]},
  }},
  {$group: { _id: null, male: {$sum: "$male"},
                        female: {$sum: "$female"},
                        total: {$sum: 1},
  }},
])

Producing given your example:

{ "_id" : null, "male" : 2, "female" : 3, "total" : 5 }

The key idea is to use a conditional expression to map the gender to 0 or 1. After that, all you need is a simple sum over each field.