Mongodb两个集合聚合

Question

球员架构：

var mongoose = require('mongoose'),Schema = mongoose.Schema;  
var playerSchema = new mongoose.Schema({  
  name: String,
  password: String,
  country: [{ type: Schema.Types.ObjectId, ref: 'countries' }]
});
mongoose.model('players', playerSchema);

国家架构：

var mongoose = require('mongoose'),Schema = mongoose.Schema;  
var countrySchema = Schema({  
  name: String,
  isActive: Boolean
});
mongoose.model('countries', countrySchema);

这是我的节点js find query：

router.route('/')
    .get(function(req, res, next) {
        mongoose.model('players').find().populate('country').exec(function (err, players) {
            console.log(players);
              if (err) {
                  return console.error(err);
              } else {
                  res.format({
                      html: function(){
                        res.render('players/index', {
                              title: 'Players',
                              "players" : players
                          });
                    },
                    json: function(){
                        res.json(players);
                    }
                });
              }
        });
    });

我想要使用countryId和name的结果集。但我得到的国家是空的。怎么办？