无法将注释对象推送到数组

Question

我无法在博客页面的评论部分发表评论：

app.post("/campgrounds/:id/comments", function(req,res){

    //lookup for the campground ID
    Campground.findById(req.params.id, function(err,campground){

        if(err){
            console.log(err);
            res.redirect("/campgrounds");
        }else{
            Comment.create(req.body.comment, function(err,comment){
                if(err){
                    console.log(err);
                }else{
                    campground.comments.push(comment);
                    campground.save();
                    res.redirect("/campgrounds/" + campground._id);
                }
            });
        }
    });
});

我得到的错误是

> (node:9300) UnhandledPromiseRejectionWarning: Unhandled promise
> rejection (rejection id: 1): ValidationError: comments: Cast to
> [undefined] failed for value
> "[{"_id":"5a6f2d7a1463e52454e367f4","text":"I wish I witness this
> serenity with my naked eyes","author":"shaykoo","__v":0}]" at path
> "comments" { comments:     [ { _id: 5a6f2d7a1463e52454e367f4,
>        text: 'I wish I witness this serenity with my naked eyes',
>        author: 'shaykoo',
>        __v: 0 } ],   _id: 5a6f2d791463e52454e367f0,   name: 'Indian camps',   image:
> 'https://farm9.staticflickr.com/8225/8524305204_43934a319d.jpg',  
> desc: 'Here is the romanian campground',   __v: 1 }

露营地架构

var mongoose= require("mongoose");

var campgroundSchema = new mongoose.Schema（{

name: String,
image: String,
desc:String,
comments: [
    {
        type :mongoose.Schema.Types.ObjectId ,
        ref:"Comment"                         //"Comment is the Model name"
    }

    ]

}）;

module.exports = mongoose.model（ “营地”，campgroundSchema）;

评论架构

var mongoose= require("mongoose");

var commentSchema = new mongoose.Schema（{

text: String,
author: String

}）;

module.exports = mongoose.model（ “注释”，commentSchema）; //这里评论是模型的单数名称