mongoose中Entity的save操作返回的不是promise吗？

环境： node v8.7.0 mongoose v4.13.9

const mongoose = require('mongoose')
mongoose.Promise = global.Promise
const db = mongoose.createConnection('localhost', 'test')

let userSchema = new mongoose.Schema({
	name:String
})
let userModel = db.model('User', userSchema)
let userEntity = new userModel({
	name:'a'
})
let save = async function(){
	await userEntity.save(()=>{
		console.log(1)
	})
	console.log(2)
}
save()

如果userEntity.save返回值是promise这里应该是先1后2，可是结果是先2后1，这是为什么？