Getting a 500 error with questions/:id/answer route

'use strict';

var express = require("express");
var app = express();
var routes = require("./routes");

var jsonParser = require("body-parser").json;
var logger = require("morgan");

app.use(logger("dev"));
app.use(jsonParser());

var mongoose = require("mongoose");

mongoose.connect("mongodb://localhost:27017/sandbox");

var db = mongoose.connection;

db.on("error", function(){
    console.log("There were problems", err);
});

db.once("open", function(){
    console.log("looks like db connection works bro");
});


app.use("/questions", routes);

// catch 404 errors
app.use(function(req, res, next){
    var err = new Error("Page Not Found");
    err.status = 404;
    next(err);
});

//Error handler
app.use(function(err, req, res, next){
    res.status(err.status || 500);
    res.json({
        error: {
            message: err.message
        }
    });
});


var port = process.env.PORT || 3000;

app.listen(port, function(){
    console.log("Express server listening on port", port);
});

'use strict';

var express = require("express");
var router = express.Router();
var Question = require("./models").Question;

router.param("id", 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);
        }
        res.question = doc;
        return next();
    });
});

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);
    }
});    

//GET /questions //this one works
router.get("/", function(req, res, next){
    //return all questions
    Question.find({})
                .sort({createdAt: -1}) 
                .exec(function(err, questions){
                    if(err) return next(err);
                    res.json(questions);
                });
    //res.json({response: "You sent a GET req"});
});

//POST /questions //this one works
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);
    });
    //return all questions
});

//GET /questions/:qid //gives 200 but doesn't show anything
router.get("/:id", function(req, res){
        res.json(req.question);
    })
    res.json({
        response: "You sent a GET req for ID "+ req.params.id
});


//POST /questions/:id/answers //500 error, doesn't work
router.post("/:id/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('/:id/answers/:aID', function(req, res){
    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('/:id/answers/:aID', function(req, res){
    req.answer.remove(function(){
        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('/:id/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 if A should be before B
    //zero to leave it unchanged
    //+ if A is to be sorted after B
    if (b.votes === a.votes){
        return b.updatedAt - a.updatedAt;
    }
    return b.votes - a.votes;
}

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

AnswerSchema.method("update", function(updates, callback){
    Object.assign(this, updates, {updatedAt: new Date()});
    this.parent().save(callback);
});

AnswerSchema.method("vote", function(vote, callback){
    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]
});

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

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

module.exports.Question = Question;