I would like to add commenting functionality to a blog web app. I currently have an article or as I call it "interview" schema as follows:

```js
var InterviewSchema = new mongoose.Schema({
  slug: {type: String, lowercase: true, unique:true},
  title: String,
  description: String,
  body: String,
  favoritesCount: {type: Number, default: 0},
  tagList: [{type: String}],
  author: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
  comments: [{type: mongoose.Schema.Types.ObjectId, ref: 'Comment'}]
}, {timestamps: true, usePushEach: true});

```

It contains an array of comments that is referenced to a comments model that I created:

```js
var CommentSchema = new mongoose.Schema({
  body: String,
  author: {type: mongoose.Schema.Types.ObjectId, ref:'User'},
  interview: {type: mongoose.Schema.Types.ObjectId, ref:'Interview'}
}, {timestamps: true});

```

In my router, I have a post method to post a comment to a particular article:

```js
router.post('/:interview/comments', auth.required, function(req, res, next){
  User.findById(req.payload.id).then(function(user) {
    if(!user) {return res.sendStatus(401); }

    var comment = new Comment(req.body.comment);
    comment.interview = req.interview;
    comment.author = user;

    return comment.save().then(function() {
      req.interview.comments.push(comment);

      return req.interview.save().then(function(article) {
        res.json({comment: comment.toJSONFor(user)});
      });
    });
  }).catch(next);
});

```

When I send a request this endpoint on postman I get the following error message:

```
{
    "errors": {
        "message": "Cannot read property 'push' of undefined",
        "error": {}
    }
}

```

Which is referencing the following line in my post method:
`req.interview.comments.push(comment);`

I can't seem to figure out why I'm getting this error message. Any suggestions or feedback is appreciated!