YelpCamp报错TypeError: 无法读取undefined的push属性

• 预期功能：将创建的评论push到每个营地（campground）实例的comments数组中
• 故障现象：注释掉Comment.create相关代码段（执行到console.log("created new comment")语句前）时应用可正常运行，取消注释执行到评论关联逻辑时报错
  

种子函数代码 #

var mongoose = require("mongoose");
var Campground = require("./models/campground");
var Comment   = require("./models/comment");
var data = [
    {
        name: "Cloud's Rest", 
        image: "https://farm4.staticflickr.com/3795/10131087094_c1c0a1c859.jpg",
        description: "blah blah blah"
    },
    {
        name: "Desert Mesa", 
        image: "https://images.unsplash.com/photo-1557214997-7eae7e0e7aaa?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxzZWFyY2h8Mnx8ZGVzZXJ0JTIwbWVzYXxlbnwwfHwwfHw%3D&auto=format&fit=crop&w=400&q=60",
        description: "blah blah blah"
    },
    {
        name: "Canyon Floor", 
        image: "https://farm1.staticflickr.com/189/493046463_841a18169e.jpg",
        description: "blah blah blah"
    }
]
function seedDB(){
    // 移除所有营地数据
    Campground.remove({}, function(err){
        if(err){
            console.log(err);
        }
        console.log("removed campgrounds!");
        // 添加初始营地数据
        data.forEach(function(seed){
            Campground.create(seed, function(err, campground){
                if(err){
                    console.log(err)
                } else {
                    console.log("added a campground");
                    // 创建关联评论
                    Comment.create(
                        {
                            text: "This place is great, but facilities are not good",
                            author: "John Paul"
                        }, function(err, comment){
                            if(err){
                                console.log(err);
                            } else {
                                campground.comments.push(comment);
                                campground.save();
                                console.log("Created new comment");
                            }
                        });   
                }
            });
        });
    }); 
}
module.exports = seedDB;

Campground模型定义代码 #

var mongoose = require("mongoose");

// Schema定义
var campgroundSchema = new mongoose.Schema({
    name: String,
    image: String,
    description: String,
    comments: [
        {
            type: mongoose.Schema.types.ObjectId,
            ref: "Comment"
        }
    ]
});

module.exports = mongoose.model("Campground", campgroundSchema);

报错核心原因是Schema定义里的类型写法错误：Mongoose中引用ObjectId类型的正确属性是大写T开头的mongoose.Schema.Types.ObjectId，你写成了全小写的mongoose.Schema.types.ObjectId，Mongoose无法识别这个不存在的属性，执行评论关联push操作时就会抛出类型错误。
另外你贴的Desert Mesa图片链接中&和ixid之间多了一个多余空格，虽然不触发本次报错，但建议删除避免后续图片加载异常。

修改campground.js中comments字段的type配置，将小写的types改为大写开头的Types即可，修正后的对应代码片段：

comments: [
    {
        type: mongoose.Schema.Types.ObjectId,
        ref: "Comment"
    }
]

内容的提问来源于stack exchange，提问作者Techboy