How to store threaded comments using MongoDB and Mongoose for Node.js?

What would be the best way to store comments trees in MongoDB and Mongoose? Currently I have this:

CommentableSchema = new Schema({
    ...
});

ReplySchema = new Schema({
    userId: Number,
    'body': String,
    createdAt: Date,
    updatedAt: Date
});

CommentSchema = new Schema({
    commentable: CommentableSchema,
    userId: Number, // users are NOT stored in MongoDB
    subject: String,
    'body': String,
    createdAt: Date,
    updatedAt: Date,
    replies: [ReplySchema],
    length: Number // comment + all replies
});

But this seems to be suitable only for top level comment + 1 more level of comments. I am fairly certain that I can not use ReplySchema inside ReplySchema.