var db = mongoose.createConnection(mongodb);
var db = mongoose.connect(mongodb);

两种方式对比：

var mongoose = require('mongoose');
db = mongoose.createConnection('localhost', 'test');
var schema = new mongoose.Schema({
 name: String
 });
var collectionName = 'kittens';
var M = db.model('Kitten', schema, collectionName);
var silence = new M({ 
name: "Silence"
});
silence.save(function(err){
 });


var mongoose = require('mongoose');
db = mongoose.connect('mongodb://localhost/test');
db.once('open', function callback () {
 // yay!
});
var kittySchema = mongoose.Schema({ name: String});
var Kitten = mongoose.model('kitten', kittySchema);
var silence = new Kitten({ name: "Silence"});
silence.save(function(err){
});

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