var mongoose = require('mongoose'),
    Schema = mongoose.Schema,
    moment = require('moment');


var OrderDetailSchema = new Schema({
    category_id: {type: String},
    category_name: {type: String},
    product_id: {type: String},
    product_name: {type: String},
    product_price: {type: Number},
    product_qty: {type: Number}
});

mongoose.model('OrderDetail', OrderDetailSchema);

var OrderSchema = new Schema({
    no: {type: String, unique: true},
    customer_id: {type: String},
    customer_name: {type: String},
    customer_tel: {type: String},
    customer_address: {
        street: String,
        city: String,
        country: String},
    details: [OrderDetailSchema],
    memo: {type: String},
    create_at: {type: Date, default: Date.now}
});

OrderSchema.virtual('order_date').get(function () {
    console.log('1233333333333333333333333');
    console.log(this.create_at);
    return moment(this.create_at).format('YYYY-MM-DD');
});

OrderSchema.virtual('total').get(function () {
    var sum = 0;
    console.log('1233333333333333333333333');
    return sum;
});

mongoose.model('Order', OrderSchema);
\n```
代码如上，查询的时候查不出设置的order_date和total属性，console的内容也没输出，是什么原因，mongoose还有什么特殊的配置吗？