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

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

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

有两种方式打开与 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()返回全局的mongoose实例