Mongoose/node error: Cannot read property 'ObjectId' of undefined
<p>I am creating a small node/express/mongo app, which allows users to post cat photos and comment on them. I have two models, <code>cat</code>, and <code>comment</code> . Everything was working fine until I decided to associate the two models together, which then caused this error:</p>

<pre><code>type: mongoose.Schema.Type.ObjectId,
                            ^ 
TypeError: Cannot read property 'ObjectId' of undefined
</code></pre>

<p>The error is referring to the cat model: </p>

<pre><code>var mongoose = require('mongoose');


var catModel = mongoose.Schema({
    name: String,
    image: String,
    owner: String,  
    description: String,
    comments: [

        {
            type: mongoose.Schema.Type.ObjectId,
            ref: "Comment"

        } 
    ]
});

var Cat = mongoose.model("Cats", catModel);

module.exports = Cat;
</code></pre>

<p>Here is the comment model :</p>

<pre><code>var mongoose = require('mongoose');

var commentSchema = mongoose.Schema({

    username: String,
    content: String,
});


Comment = mongoose.model('Comment', commentSchema);

module.exports = Comment;
</code></pre>

<p>Here is a snippet of app.js :</p>

<pre><code>var express = require('express');
var app = express();
//more modules
var Comment = require('./models/comment.js');
var Cat = require('./models/cat.js');


//home route 

app.get('/cats', function(req,res) {

    Cat.find({}, function(err, cats) {
        if (err) {
            console.log(err);

        } else {
          res.render('cats', {cats: cats});  
        }  
    })
});
</code></pre>

<p>I am using mongoose 4.3.7 . I researched this problem and couldn't solve it. For example, I looked at <a href="https://stackoverflow.com/questions/7936845/mongoose-error-cannot-read-property-objectid-of-undefined">this</a> post and re-installed mongoose, but the problem persisted.</p>