Mongoose与其他数据有很多很多关系

Question

产品架构：

var ProductSchema = new Schema({
    productName: {type: String, required: true},
});

客户架构：

var ClientSchema = new Schema({
    clientName: {type: String, required: true},
    products: [{ type : mongoose.Schema.ObjectId, ref : 'Product'}],
});

如何在关系中添加其他字段？例如，添加字段数量：

var ClientSchema = new Schema({
    clientName: {type: String, required: true},
    products: [{ type : mongoose.Schema.ObjectId, ref : 'Product', quantity: Number}],
});