parallel count query in one model for single params

gistfile1.txt

// Use MongoDB 3.0  group() function for getting users and count attribute

function countUsersAttributes(_key, _cond) {
    var deferred = Q.defer();
    db_mongo_driver.users.group({
        key: _key,
        cond: _cond,
        reduce: function (value, result) {
            result.count++;
        },
        initial: {
            count: 0
        }
    }, function (err, countObj) {
        if (err || !countObj) {
            deferred.reject(err || {
                error: 'no results'
            });
        }
        deferred.resolve(countObj[0])
    });
    return deferred.promise
}


// Prepare list of promises 

  var arrayPromises = [];
    for (var i = 0; i < listOfQuerys.length; i++) {
        arrayPromises.push(countUsersAttributes(listOfQuerys[i].key, listOfQuerys[i].cond))
    }
    Q.all(arrayPromises).then(function (err, _res) {
        res.status(200).jsonp(err || _res);
    });
    
 // Use this array for add the field that you need to count, and  condiction about this field  if you need that 
 
 var listOfQuerys = [
    {
        key: {
            gender: 1
        },
        cond: {
            gender: {
                $exists: true
            }
        }
    },
    {
        key: {
            userAgent: 1
        },
        cond: {
            userAgent: {
                $exists: true
            }
        }
    }, {
        key: {
            phoneNumber: 1
        },
        cond: {}
    }
];

Result example:

[
  {
    "gender": "male",
    "count": 1
  },
  {
    "userAgent": "Other",
    "count": 1
  },
  {
    "phoneNumber": null,
    "count": 2
  }
]