MapReduce: count sites per country

sites-map-reduce.js

db.sites.insert({"cities": [ { "country" : { "code" : "ES" } } ] });
db.sites.insert({"cities": [ { "country" : { "code" : "US" } } ] });
db.sites.insert({"cities": [ { "country" : { "code" : "US" } } ] });
db.sites.insert({"cities": [ { "country" : { "code" : "ES" } } ] });
db.sites.insert({"cities": [ { "country" : { "code" : "PT" } } ] });
db.sites.insert({"cities": [ { "country" : { "code" : "BR" } } ] });
db.sites.insert({"cities": [ { "country" : { "code" : "BR" } } ] });
db.sites.insert({"cities": [ { "country" : { "code" : "CA" } } ] });
db.sites.insert({"cities": [ { "country" : { "code" : "CA" } } ] });
db.sites.insert({"cities": [ { "country" : { "code" : "CA" } } ] });


db.sites.mapReduce(
  function() { emit(this.cities[0].country.code, 1) },
  function(key, values) {
    var c = 0;
    values.forEach(function(i) { c += i; });
    return c;
  },
  {out: { inline: 1 } }
);