Node.js + 无法读取未定义的属性 'query'

Question

我是 Node.js 的新手，想将我的示例数据库与节点 js 连接并获取结果。我已经完成了以下步骤。

在我的 Mac
安装MySQLWorkbench
创建数据库，table 和 table 中的一些行。
通过关注 this.
安装 Atom。
通过 npm install mysql --save

应用程序运行在没有 SQL 查询的情况下很好，我能够在本地主机上看到消息，但是在添加 SQL 连接时，它会产生问题并显示错误

Cannot read property 'query' of undefined.

app.js 的内容：

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');
var mysql = require("mysql");
var bodyParser = require('body-parser');
var mysql= require('mysql');
var http = require('http');

var indexRouter = require('./routes/api/v1/index');
var usersRouter = require('./routes/api/v1/users');

var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'pug');
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use('/', indexRouter);
app.use('/api/v1/users', usersRouter);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  next(createError(404));
});

//Database connection
app.use(function(req, res, next){
  console.log('In');
    res.locals.connection = mysql.createConnection({
        host     : '127.0.0.1',
        user     : 'root',
      password : 'q1w2e3r4t5',
        database : 'users'
    });
    res.locals.connection.connect();
  console.log('out');
    next();
});


// error handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;
var server = http.createServer(app);
server.listen(4001);

users.js 的内容：

router.get('/', function(req, res, next) {
res.locals.connection.query('SELECT * from userInfo', function (error, results, fields) {
        if(error){
            res.send(JSON.stringify({"status": 500, "error": error, "response": null}));
            //If there is error, we send the error in the error section with 500 status
        } else {
            res.send(JSON.stringify({"status": 200, "error": null, "response": results}));
            //If there is no error, all is good and response is 200OK.
        }
    });
});

Answer 1

您需要在声明路由之前声明 app.use 个方法。

参考：express docs

来自文档：

Middleware functions are executed sequentially, therefore the order of middleware inclusion is important.

Answer 2

尝试在指定user.js的路由之前把连接数据库的代码放上去：

    // view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'pug');
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

//Database connection
app.use(function(req, res, next){
  console.log('In');
    res.locals.connection = mysql.createConnection({
        host     : '127.0.0.1',
        user     : 'root',
      password : 'q1w2e3r4t5',
        database : 'users'
    });
    res.locals.connection.connect();
  console.log('out');
    next();
});


app.use('/', indexRouter);
app.use('/api/v1/users', usersRouter);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  next(createError(404));
});



// error handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;
var server = http.createServer(app);
server.listen(4001);

Answer 3

您尝试使用它的方式是作为中间件。这意味着每次调用 api 时，都会建立一个新的数据库连接。当有很多并行请求时，这将与数据库建立很多连接。理想情况下，您应该只创建一个连接并在整个应用程序中使用它。

创建一个新文件dbconnection.js

//create connection    
let connection = mysql.createConnection({
        host     : '127.0.0.1',
        user     : 'root',
        password : 'q1w2e3r4t5',
        database : 'users'
});
//export connection, you can optionally use 
//connection.connect here. The same state of this module is passed everywhere. 
exports.connection = connection;

现在在您的 app.js 中，使用 var connection = require('./dbconnection'); 导入它，这将自动创建一个连接。

您现在可以使用您的中间件在 res.locals 中设置连接，但请记住在您的路由文件之前使用它。

app.use(function(req, res, next){
  console.log('In');
  res.locals.connection = connection
  console.log('out');
  next();
});

...

app.use('/', indexRouter);
app.use('/api/v1/users', usersRouter);

这应该可以完成工作。

Node.js + 无法读取未定义的属性 'query'

Node.js + Cannot read property 'query' of undefined

mysql

api

localhost

node.js

atom-editor

Node.js + 无法读取未定义的 属性 'query'

Node.js + Cannot read property 'query' of undefined

mysql

api

localhost

node.js

atom-editor

Node.js + 无法读取未定义的属性 'query'