Node.js中Mongoose查询事件顺序异常问题求助

我通过Mongoose将Node.js应用连接至MongoDB，需要检查数据库中是否存在标题为指定topic的数据。我使用find()方法遍历数据逐一与topic比对，若未找到则返回特定标识，但实际先触发了“未找到”的代码。通过console.log发现日志先输出default，再输出checking in posts，而非预期顺序。

代码片段 #

app.get("/posts/:topic", (req, res) => {
  const topic = _.lowerCase(req.params.topic);
  var found = false;

  posts.find().then((result)=>{
    result.forEach((element)=>{
      if(_.lowerCase(element.postTitle) === topic){
        found = true;
        console.log("checking in posts..");
        res.render("post", {postTitle: element.postTitle, postContent: element.postContent});
      }
    })
  }).catch((err)=>console.log(err));
  

  if(found == false){
      console.log("default");

      // res.render("post", {postTitle: "Oops!...", postContent: "The post you are searching, does not exists.....try again."});
  }

});

日志输出 #

default
checking in posts

求助如何修正事件执行顺序，让查询逻辑先执行再判断是否返回未找到标识。

问题根源是Mongoose的find()是异步操作，它不会阻塞后续代码执行。你的if(found == false)代码会在数据库查询完成前就执行，所以此时found还是初始的false，先输出了default，之后数据库查询完成才进入then回调输出checking in posts。

可以通过以下方式修复：

方式一：将判断逻辑移至then回调内部 #

把未找到的处理逻辑放到find()的then回调里，确保在数据库查询完成后再执行判断：

app.get("/posts/:topic", (req, res) => {
  const topic = _.lowerCase(req.params.topic);

  posts.find().then((result)=>{
    let found = false;
    result.forEach((element)=>{
      if(_.lowerCase(element.postTitle) === topic){
        found = true;
        console.log("checking in posts..");
        res.render("post", {postTitle: element.postTitle, postContent: element.postContent});
      }
    });
    // 遍历完结果后再判断是否未找到
    if(!found){
      console.log("default");
      res.render("post", {postTitle: "Oops!...", postContent: "The post you are searching, does not exists.....try again."});
    }
  }).catch((err)=>{
    console.log(err);
    // 异常情况下返回错误页面
    res.render("post", {postTitle: "Error", postContent: "Something went wrong."});
  });
});

方式二：使用async/await简化异步逻辑 #

用async/await让异步代码看起来像同步代码，更易读：

app.get("/posts/:topic", async (req, res) => {
  const topic = _.lowerCase(req.params.topic);
  try {
    const result = await posts.find();
    let found = false;
    result.forEach((element)=>{
      if(_.lowerCase(element.postTitle) === topic){
        found = true;
        console.log("checking in posts..");
        res.render("post", {postTitle: element.postTitle, postContent: element.postContent});
      }
    });
    if(!found){
      console.log("default");
      res.render("post", {postTitle: "Oops!...", postContent: "The post you are searching, does not exists.....try again."});
    }
  } catch (err) {
    console.log(err);
    res.render("post", {postTitle: "Error", postContent: "Something went wrong."});
  }
});

额外优化建议 #

不需要用find()查询所有数据再遍历比对，直接在数据库层面过滤效率更高：

app.get("/posts/:topic", async (req, res) => {
  const topic = _.lowerCase(req.params.topic);
  try {
    // 直接查询匹配标题的文档，用正则实现不区分大小写匹配
    const post = await posts.findOne({
      postTitle: { $regex: new RegExp(`^${topic}$`, "i") }
    });
    if(post){
      console.log("checking in posts..");
      res.render("post", {postTitle: post.postTitle, postContent: post.postContent});
    } else {
      console.log("default");
      res.render("post", {postTitle: "Oops!...", postContent: "The post you are searching, does not exists.....try again."});
    }
  } catch (err) {
    console.log(err);
    res.render("post", {postTitle: "Error", postContent: "Something went wrong."});
  }
});

这样避免了把所有数据拉到服务器再遍历，提升了性能，尤其是数据量大的时候。

内容的提问来源于stack exchange，提问作者navneet sharma