I can't get vote-up or down to work. This.parent(This isn't getting sent in the parameters)

'use strict';
var express = require('express');
var router = express.Router();
var Question = require("./models.js").Question;

//questions param https://expressjs.com/en/api.html#app.param
router.param("qID", function(req,res,next,id){
    Question.findById(id, function(err, doc){
        if(err) return next(err);
        if(!doc){
            err = new Error("Not Found");
            err.status = 404;
            return next(err);
        }
        req.question = doc;
        return next();
    });

    //answers
router.param("aID", function(req,res,next,id){
    req.answer = req.question.answers.id(id);
    if(!req.answer){
        err = new Error("Not Found");
        err.status = 404;
        return next(err);
    }
    next(); 
});


});

//GET /questions
//route for getting questions
router.get("/", function(req,res,next){
        //  This is an alternative way to make a querey to get the questions and then sort them
    //Question.find({}, null, {sort: {createdAt: -1}}, function(err, questions){
    //     if(err) return next(err);
    //     res.json(questions);

    // });

    //Query builder 
    Question.find({})
        .sort({createdAt: -1})
        .exec(function(err, questions){
            if(err) return next(err);
            res.json(questions);
        });
});

//POST /questions
//Route for creating questions
router.post("/", function(req,res,next){
    var question = new Question(req.body);
    question.save(function(err, question){
        if (err) return next(err);
        res.status(201);
        res.json(question);

    });
});

//GET /questions
//route for getting a specfic question
router.get("/:qID", function(req,res,next){
        res.json(req.question);
});



//POST /questions/:id/answer
//Route for creating an answer
router.post("/:qID/answers", function(req,res,next){
    req.question.answers.push(req.body);
    req.question.save(function(err, question){
        if(err) return next(err);
        res.status(201);
        res.json(question);
    });
});

//PUT /questions/:qID/answers/:aID
//Edit a specific answer
router.put("/:qID/answers/:aID", function(req,res,next){
    req.answer.update(req.body, function(err, result){
        if(err) {return next(err)};
        res.json(result);
    });

});

//DELETE /questions/:qID/answers/:aID
//Delete a specific answer
router.delete("/:qID/answers/:aID", function(req,res,next){
    req.answer.remove(function(err){
        req.question.save(function(err, question){
            if(err){ return next(err)};
            res.json(question);
        });

    });

});


//POST /questions/:qID/answers/:aID/vote-up
//POST /questions/:qID/answers/:aID/vote-down
//Vote on a specific answer
router.post("/:qID/answers/:aID/vote-:dir", 
    function(req, res, next){
        if(req.params.dir.search(/^(up|down)$/) === -1) {
            var err = new Error("Not Found");
            err.status = 404;
            next(err);
        } else {
            req.vote = req.params.dir;
            next();
        }
    }, 
    function(req, res, next){
        req.answer.vote(req.vote, function(err, question){
            if(err) return next(err);
            res.json(question);
        });
});


module.exports = router; 

'use strict';

var mongoose = require('mongoose');

var Schema = mongoose.Schema;

var sortAnswers = function(a, b){
    // - negative a before b
    // 0 no change
    // + positive if a before b
    if(a.votes === b.votes){
        return b.updatedAt - a.updatedAt;
        // if(a.updatedAt > b.updatedAt){
        //     return -1;
        // }else if(a.updatedAt < b.updatedAt){
        //     return 1
        // }else{
        //     return 0;
        // }
    }
    return b.votes - a.votes;
};

var AnswerSchema = new Schema({
    text: String,
    createdAt: {type: Date, default: Date.now},
    updatedAt: {type: Date, default: Date.now},
    votes: {type: Number, default: 0}

});

AnswerSchema.method("update", (updates, callback) => {

    Object.assign(this, updates, {updatedAt: new Date()});
    this.parent().save(callback);

});

AnswerSchema.method("vote", (vote, callback)=>{
console.log(this)
    if(vote ==="up"){
        this.votes += 1;
    }else{
        this.votes -= 1;
    }

    this.parent().save(callback);

});

var QuestionSchema = new Schema({
    text: String,
    createdAt: {type: Date, default: Date.now},
    answers: [AnswerSchema]
});

//call save hook 
QuestionSchema.pre("save", function(next){
    this.answers.sort(sortAnswers);
    next();

});

var Question = mongoose.model("Question", QuestionSchema);

module.exports.Question = Question;