Mongoose adding new entry to parent

I'm building a REST API with express and mongoose. I have the following schemas :

var PostSchema= new Schema({
name: String,
_comments: [{ type: Schema.Types.ObjectId, ref: 'Comment'}]
});

var CommentSchema = new Schema({
text: String,
_post: { type: Schema.Types.ObjectId, ref: 'Post'}
});

I want to add a new comment to my post :

/* POST a comment*/
router.post('/', function(req, res, next) {
var comment= new Comment(req.body);
comment.save(function (err, post) {
    if (err) {
        return next(err);
    }
    res.json(post);
});
});

It does save the comment with the following data :

{text: "bla", _post: *postId*}

However, when I retrieve my posts with populated comments :

/* GET post*/
router.get('/', function(req, res, next) {
Post.find().populate('_comments').exec(function (err, posts) {
    if (err) return next(err);
    res.json(posts);
});
});

The comments array is empty.

So I guess that when I'm adding a new comment to a post, I also need to add the comment id to the post.comments array and save it ? Is there a clean way to do that ?