Lambda函数更新MongoDB时出现[object Promise]错误求解决

运行Lambda函数从数组随机选取对象更新MongoDB唯一文档时，返回如下错误：

{
  "errorType": "object",
  "errorMessage": "[object Promise]",
  "trace": []
}

以下是我的代码：

const MongoClient = require("mongodb").MongoClient;

const MONGODB_URI =
  "Removed for Privacy";

let cachedDb = null;

async function connectToDatabase() {
  if (cachedDb) {
    return cachedDb;
  }

  // Connect to our MongoDB database hosted on MongoDB Atlas
  const client = await MongoClient.connect(MONGODB_URI);

  // Specify which database we want to use
  const db = await client.db("redacted");

  cachedDb = db;
  return db;
}


exports.handler = async (event, context, callback) => {

  context.callbackWaitsForEmptyEventLoop = false;

  const db = await connectToDatabase();

    const WORDS = [ { "word": 'exampleWord', "description": 'exampleDescription' }]
    
    let wotd = WORDS[Math.floor(Math.random() * WORDS.length)]
    
    
    let updateWord = db.collection('wordoftheday').updateOne({"_id":"ObjectId('redacted')"},{$set: {
      "word": wotd.word,
      "description": wotd.description}})
  
    callback(updateWord)
    
    };

错误原因 #

1. 未等待异步操作完成：db.collection.updateOne()是异步方法，执行后会返回一个Promise。直接将这个Promise传给callback，Lambda无法解析Promise对象，就会抛出[object Promise]错误。
2. _id格式错误：把"ObjectId('redacted')"作为字符串传入查询条件，MongoDB不会将其识别为有效的ObjectId类型，导致更新无法匹配到目标文档。

修复后的代码 #

const MongoClient = require("mongodb").MongoClient;
// 引入ObjectId构造函数处理_id
const { ObjectId } = require("mongodb");

const MONGODB_URI = "Removed for Privacy";

let cachedDb = null;

async function connectToDatabase() {
  if (cachedDb) {
    return cachedDb;
  }

  const client = await MongoClient.connect(MONGODB_URI);
  const db = client.db("redacted"); // client.db是同步方法，无需await

  cachedDb = db;
  return db;
}


exports.handler = async (event, context) => {
  context.callbackWaitsForEmptyEventLoop = false;

  try {
    const db = await connectToDatabase();
    const WORDS = [ { "word": 'exampleWord', "description": 'exampleDescription' }];
    const wotd = WORDS[Math.floor(Math.random() * WORDS.length)];
    
    // 等待updateOne执行完成，同时正确转换_id类型
    const updateResult = await db.collection('wordoftheday').updateOne(
      {"_id": ObjectId('redacted')},
      {$set: {
        "word": wotd.word,
        "description": wotd.description
      }}
    );

    // async handler直接返回结果即可，无需callback
    return {
      statusCode: 200,
      body: JSON.stringify({
        message: "更新成功",
        matchedCount: updateResult.matchedCount,
        modifiedCount: updateResult.modifiedCount
      })
    };
  } catch (error) {
    return {
      statusCode: 500,
      body: JSON.stringify({
        error: error.message
      })
    };
  }
};

关键修改点 #

• 引入ObjectId构造函数，将字符串形式的_id转换为MongoDB可识别的类型。
• 使用await等待updateOne异步操作完成，获取实际的更新结果。
• 利用async函数特性直接返回结果，替代传统callback写法（Lambda对async handler的支持更简洁）。
• 添加try-catch块捕获异常，返回更清晰的错误信息。

内容的提问来源于stack exchange，提问作者mannymcfly