比如有如下Schema

var Account = new Schema({
    username: String,
    password: String,
    Email: String,
    applyform: [{ type: ObjectId, ref: 'Applyform' }]
});

var Applyform = new Schema({
    classid : [{ type: ObjectId, ref: 'Classinfo' }],
    price: Number,
    applydate: { type: Date, default: Date.now},
});

var Classinfo = new Schema({
    classname: String,
    createdate: { type: Date, default: Date.now }
});

我现在对Acount进行查找并populate

Account
.findOne({username: 'someone'})
.populate('applyform')
.exec(function (err, res) {
.....
})

结果res.applyform可以读到res.applyform.classid 但我想继续populate读到classname可以么？应该怎么继续populate？