问使用mongoose和express js的mongodb数据库出现问题，下面是教程

EN

import express from 'express';
import cors from 'cors';
import bodyParser from 'body-parser';
import mongoose from 'mongoose';
import Issue from './models/Issue';

const app = express();
const router = express.Router();

app.use(cors());
app.use(bodyParser.json());

mongoose.connect('mongodb://localhost:27017/issues', { useNewUrlParser: true });

const connection = mongoose.connection;

connection.once('open',  () =>{
    console.log("MongoDB connection established successfully");
});


router.route('/issues').get( (req, res) => {
    Issue.find((err,issues) => {
        if(err)
            console.log(err);
        else
            res.json(issues); 
    });
});

import mongoose from 'mongoose';

const Schema = mongoose.Schema;

let Issue = new Schema ({

    title: {
        type: String
    },

    responsible: {
        type: String
    },

    description: {
        type: String
    },

    severity: {
        type: String
    },

    status: {
        type: String,
        default: 'Open'
    }

});

export default mongoose.model('Issue', Issue);

ls -ail
total 12
656701 drwxrwxrwx  3 root root 4096 oct 15 18:56 .
     2 drwxr-xr-x 25 root root 4096 oct 15 18:56 ..
658216 drwxrwxrwx  4 dan  root 4096 oct 16 11:29 db