The `useMongoClient` Option! I Need help setting it up?

'use strict';

var mongoose = require("mongoose");
mongoose.Promise = global.Promise;
var promise = mongoose.connect("mongodb://localhost:27017/sandbox", {
    useMongoClient: true
});
var db = mongoose.connection;

db.on("error", function(err) {
    console.error("connection error:", err);
});

db.once("open", function() {
    console.log("db connection successful");
    // All database communication goes here

var Schema = mongoose.Schema;
var AnimalSchema = new Schema({
type: { type:String, default: "goldfish"},
size: { type:String, default: "small"},
color: { type: String, default: "golden"},
mass: { type:Number, default: 0.007},
name: { type:String, default: "angela"}
});

var Animal = mongoose.model("Animal", AnimalSchema);

var elephant = new Animal({
    type: "elephant",
    size: "big",
    color: "gray",
    mass: 6000,
    name: "lawrence"
});

var animal = new Animal({}); // Goldfish

Animal.remove({}), function(){
    if (err) console.error("Save Failed.", err);
    elephant.save(function(err){
        if (err) console.error("Save Failed.", err);
        animal.save(function(err){
                if (err) console.error("Save Failed.", err);
                db.close(function(){
                    console.log("db connection closed");
});

});
});
};
});

'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());

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

// catch 404 and forward to error handler
app.use(function (req, res, next){
    var err = new Error("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('exress server is listening on port', port);
});

'use strict';

var express = require('express');
var router = express.Router();

//GET /questions
// Route for questions collection
router.get("/", function (req, res) {
        res.json({response: "You sent me a GET request"});
});

//POST /questions
// Route for creating questions
router.post("/", function (req, res) {
        res.json({
            response: "You sent me a POST request",
        body: req.body
        });
});

//GET /questions
// Route for specific questions
router.get("/:qID", function (req, res) {
        res.json({
            response: "You sent me a GET request for ID " + req.params.qID
        });
});

//POST /questions/:id/answers
// Route for creating an answer
router.post("/:qID/answers", function (req, res) {
        res.json({
            response: "You sent me a POST request to /answers",
            questionId: req.params.qID,
            body: req.body
        });
});

//PUT /questions/:qID/answers/:aID
// Edit a specific answer
router.put("/:qID/answers/:aID", function(req, res) {
        res.json({
            response: "You sent me a PUT request to /answers",
            questionId: req.params.qID,
            answerId: req.params.aID,
            body: req.body
        });
});

//Delete /questions/:qID/answers/:aID
// Delete a specific answer
router.delete("/:qID/answers/:aID", function(req, res) {
        res.json({
            response: "You sent me a Delete request to /answers" + req.params.dir,
            questionId: req.params.qID,
            answerId: req.params.aID,
        });
});

//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 {
    next();
}
}, function(req, res) {
        res.json({
            response: "You sent me a POST request to /vote-" + req.params.dir,
            questionId: req.params.qID,
            answerId: req.params.aID,
            vote: req.params.dir
        });
});

module.exports = router;