node.js - 为Schema创建了一个speak方法，为什么之后的代码没法调用这个speak方法？

Question

// getting-started.js
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/test');

var db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function() {
  // we're connected!
});

    var kittySchema = mongoose.Schema({
        name: String
    });

var Kitten = mongoose.model('Kitten', kittySchema);

var silence = new Kitten({ name: 'Silence' });
console.log(silence.name); // 'Silence'

// NOTE: methods must be added to the schema before compiling it with mongoose.model()
kittySchema.methods.speak = function () {
    var greeting = this.name
    ? "Meow name is " + this.name
    : "I don't have a name";
    console.log(greeting);
}

var Kitten = mongoose.model('Kitten', kittySchema);

var fluffy = new Kitten({ name: 'fluffy' });
fluffy.speak(); // "Meow name is fluffy"

fluffy.save(function (err, fluffy) {
    if (err) return console.error(err);
    fluffy.speak();
});

Kitten.find(function (err, kittens) {
    if (err) return console.error(err);
    console.log(kittens);
})

Kitten.find({ name: /^fluff/ }, callback);

---

错误如下：