在自己的架构中使用架构属性？猫鼬nodejs

Question

有一种方法可以在其自己的模型中使用架构属性

    var mongoose = require("mongoose");

//====================================================
// Schema 
//====================================================

var bookSchema = new mongoose.Schema({
    name: String,
    image: String,
    summary: String,
    author: String,
    genre: String,
    publisher: String,
    available: Number,
    submitted: {
        id: {
            type: mongoose.Schema.Types.ObjectId,
            ref: "User" //model name
        },
        username: String
    },
    reviews: [
        {
            type: mongoose.Schema.Types.ObjectId,
            ref: "Review"
        }
        ],
    reserves: {
        type: [{
            type: mongoose.Schema.Types.ObjectId,
            ref: "Reserve" 
        }],
        validate: [arrayLimit, '{PATH} exceeds the limit of 5']
    }
});


function arrayLimit(val) {
    return val.length <= 5;
}

module.exports = mongoose.model("Book", bookSchema);

函数arraylimit中的

。我正在尝试将bookSchema中的“ 5”替换为“ available”属性。