部署在Heroku的MERN应用无法获取MongoDB数据求助

本地环境下MERN应用与MongoDB均运行正常，但部署到Heroku后，其他设备访问网页时出现数据错误，报错信息：

Failed to load resource: net:: ERR_Connection_Refused
localhost:8000/api/ads:1

已完成的配置 #

• MongoDB Atlas设置通用网络访问为0.0.0.0/0
• Heroku中配置了环境变量

相关代码与环境变量 #

server.js代码 #

const express = require('express');
const mongoose = require('mongoose');
const cors = require('cors');
const path = require('path');
const session = require('express-session');
const dotenv = require('dotenv');
const MongoStore = require('connect-mongo');
const bcrypt = require('bcryptjs');

// set dotenv
dotenv.config();

// import routes
const noticesRoutes = require('./routes/notices.routes');
const authRoutes = require('./routes/auth.routes');

const app = express();

/* MIDDLEWARE */

app.use(cors());
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(session({ 
  secret: bcrypt.hashSync('pass', 8), 
  store: MongoStore.create({ mongoUrl: process.env.DB_URL, collection: 'sessions' }), 
  resave: false, 
  saveUninitialized: false,
  cookie: {
    secure: process.env.NODE_ENV == 'production',
  }, 
}))

// access to storage folder
app.use(express.static(path.join(__dirname, '/client/build')));
app.use(express.static(path.join(__dirname, '/public')));
app.use(express.static(path.join(__dirname, '/uploads/')));

// use routes
app.use('/api', noticesRoutes);
app.use('/auth', authRoutes);

// error status
app.use((req, res) => {
  res.status(404).send('404 not found...');
})

// React website
app.get('*', (req, res) => {
  res.sendFile(path.join(__dirname + '/client/build/index.html'));
});

/* MONGOOSE */
const NODE_ENV = process.env.NODE_ENV;
let dbUri = '';
const password = process.env.PASSDB;

if(NODE_ENV === 'production') dbUri = 'mongodb+srv://MarcinEden:' + password + '@cluster1.sg1w6lh.mongodb.net/RealEstateBoard?retryWrites=true&w=majority';
else dbUri = 'mongodb+srv://MarcinEden:' + password + '@cluster1.sg1w6lh.mongodb.net/RealEstateBoard?retryWrites=true&w=majority';

mongoose.connect(dbUri, { useNewUrlParser: true, useUnifiedTopology: true });
const db = mongoose.connection;

db.once('open', () => {
  console.log('Successfully connected to the database');
});
db.on('error', err => console.log('Error: ' + err));

/* START SERVER */
const port = process.env.PORT || 8000;
app.listen(port, () => {
  console.log('Server is running on port: '+ port);
});

.env文件环境变量 #

NODE_ENV = production
DB_URL = mongodb+srv://MarcinEden:pass@cluster1.sg1w6lh.mongodb.net/RealEstateBoard?retryWrites=true&w=majority

解决思路 #

1. 修正前端API请求地址
   报错显示请求指向localhost:8000/api/ads，说明前端硬编码了本地地址。生产环境需改为动态地址：

   • 开发环境用http://localhost:8000
   • 生产环境用相对路径（或空字符串），让请求自动指向Heroku应用域名

2. 补全缺失的环境变量
   server.js中用到了process.env.PASSDB，但当前.env和Heroku配置中未设置该变量，需在Heroku控制台添加PASSDB变量，值为MongoDB Atlas的数据库密码

3. 优化MongoDB连接逻辑
   代码中同时存在DB_URL环境变量和手动拼接连接字符串的逻辑，冗余且易出错。建议直接使用process.env.DB_URL作为连接地址：

   mongoose.connect(process.env.DB_URL, { useNewUrlParser: true, useUnifiedTopology: true });
   

4. 验证Heroku环境变量生效状态
   在终端执行heroku config，确认NODE_ENV、DB_URL、PASSDB三个变量均已正确配置

5. 调整CORS配置（可选）
   生产环境可限定CORS origin为Heroku应用域名，避免跨域问题：

   app.use(cors({
     origin: process.env.NODE_ENV === 'production' ? 'https://你的Heroku应用域名.herokuapp.com' : 'http://localhost:3000',
     credentials: true
   }))
   

内容的提问来源于stack exchange，提问作者MarcinB