express - Express Mongoose DB.once ('open') 无法执行回调函数

Question

exports.c_39 = function(req,res) {
    var mongoose = require('mongoose');
    mongoose.createConnection('mongodb://localhost/cj');
    var db = mongoose.connection;
    db.on('error', console.error.bind(console, 'connection error:'));
    console.log('a')
    db.once('open',function(){
        console.log('b')
    })
}

可以执行console.log('a')，但不能执行DB.once('open')回调函数

Answer 1

那是因为mongoose.connection与返回的连接不同createConnection()。

有两种方法可以打开与 Mongoose 的连接：

// method 1: this sets 'mongoose.connection'
> var client = mongoose.connect('mongodb://localhost/test');
> console.log(client.connection === mongoose.connection)
true

// method 2: this *doesn't* set 'mongoose.connection'
> var connection = mongoose.createConnection('mongodb://localhost/test');
> console.log(client.connection === mongoose.connection)
false

因此，要解决您的问题，您需要将事件处理程序连接到由返回的连接createConnection()，而不是mongoose.connection：

var db = mongoose.createConnection('mongodb://localhost/cj');
db.once('open', function() { ... });

简而言之：

• .createConnection()返回一个Connection实例
• .connect()返回全局mongoose实例

Answer 2

而不是mongoose.createConnection使用： mongoose.connect('mongodb://localhost/cj');

Answer 3

做

db=  mongoose.createConnection('mongodb://localhost/cj'). 

然后使用

db.on("connected", connected); with connected = err => {
  console.log(`connected ${db.readyState}`);
};