Is it possible to list nonexisting groups when using $group? - node.js

Question

If I want to perform a $group and $sum on a mongodb collection from my node server (using mongoosoe), is it possible to return 0 for the non existing groups?
the collection has the following fields: ssn, name, gender, city.
model.aggregate([
{
$group : { _id : { city:"$city", gender:"$gender"}, count{ $sum:1 }}
}], function (err,result) {
if(err) {
//err
}
else{
//response
}
});
if there are people of both genders in the city - the query will return:
{
"_id" : {
"city" : "NY",
"gender" : "male"
},
"count" : 11
},
{
"_id" : {
"city" : "NY",
"gender" : "female"
},
"count" : 31
}
but if people of one gender are not present in a city - no value will be returned. for example no males in LA:
{
"_id" : {
"city" : "LA",
"gender" : "female"
},
"count" : 53
}
is it possible to make the query return the following result for given scenario without having a collection with cities and population quantities?
{
"_id" : {
"city" : "LA",
"gender" : "male"
},
"count" : 0
},
{
"_id" : {
"city" : "LA",
"gender" : "female"
},
"count" : 53
}
thanks,

Answer 1

If the possible values are finite and known as in your example, you could use $cond to combine the counts for male and female into one document per city like this:
[
{
$group : {
_id: {
city:"$city"
},
males:{
$sum: {
$cond: {if: {$eq:["$gender", "male"]}, then: 1, else: 0}
}
},
females:{
$sum: {
$cond: {if: {$eq:["$gender", "female"]}, then: 1, else: 0}
}
}
}
}
]