之前使用mongoose时候 直接在app.js中这样写

var mongoose = require('mongoose');

//db conn
var dbUrl = 'mongodb://localhost/sand';
mongoose.connect(dbUrl);
var db = mongoose.connection;
db.on('error', console.error.bind(console, 'mongodb connection error'));
db.once('open', function(callback) {
	console.log('mongodb connection right');
});

然后定义model层

var mongoose = require('mongoose');
var bcrypt = require('bcrypt');

var SALT_WORK_FACTOR = 10;
var UserSchema = mongoose.Schema({
    name: {unique: true, type: String},
    password: {type: String},
    nickname: {type: String},
    role: {type: Number, default: 0}, //0普通用户，1老师， 2管理员
    createTime: {type: Date, default: Date.now()}
});
。。。
var User = mongoose.model('User', UserSchema);

module.exports = User;

因为两个mongoose是一样的，app修改了该node空间里面缓存的mongoose，其他require时，从该缓存取出，所以都是一样