但是不知道怎么用啊，在官方也没找到相关的demo，有人写过demo测试了吗？

更新下，在mongoose官网上找到了demo

http://thecodebarbarian.com/a-node-js-perspective-on-mongodb-4-transactions.html

自己写的demo如下：

const mongoose = require('mongoose');
const Config = require('./config');
let db = mongoose.createConnection('mongodb://127.0.0.1:27013,127.0.0.1:27012,127.0.0.1:27011/test', {
		replicaSet: 'test',
		readPreference: "secondaryPreferred"
	},function(err){
		if(err){
		console.log('connect error.');
		}else{
		console.log('connect success.');
		}
});


let UserModel = db.model('UserModel', new mongoose.Schema({
  nick:String,
  money:Number
}),'user');
async function test1(){
	let doc = await UserModel.create([{nick:"a",money:10},{nick:"b",money:15}]);
}
async function test2(){
  let session = await UserModel.startSession();
  session.startTransaction();
  try{
	  let opts = {session,new:true};
	  let a = await UserModel.findOneAndUpdate({nick:"a"},{$inc:{money:-2}},opts);
	  console.log(a.toObject());
	  let b = await UserModel.findOneAndUpdate({nick:"b"},{$inc:{money:2}},opts);
	  console.log(b.toObject());
	  await session.commitTransaction();
	  session.endSession();
  }catch(err){
	  console.log(err);
	  await session.abortTransaction();
	  session.endSession();
  }
  console.log('transaction complete.');
}
test2();