为什么我无法从Mogoose的Model.find（）使用console.log输出结果？

Question

So I've been working on MongoDB with mongoose , and I've got the following code which will be include with var tdb = require('./db.js');

// ./db.js
module.exports = {
    update: function(data) {
        post.findOne({slug:data.slug}, (err, res) => {
            if(err) {
                post.create(data, (err, res) => {})
            } else {
                post.updateOne({slug:data.slug}, data, (err, res) => {})
            }
        })
    },
    search: function(ftype, fval, fkey) {
        var query = {};
        if(ftype != "all") {
            query[ftype] = fval;
        }
        post.find(query).lean().exec((err, res) => {
            var keys = Object.keys(res);
            var index = keys.indexOf(fkey);
            var rdata = res[fkey];
            rdata.next = res[keys[index + 1]];
            rdata.prev = res[keys[index - 1]];
            return rdata;
        })
    }
}

(The code also have other part, but I'm focusing on search function)

Which then I try to call console.log(tdb.search("all", "none", "2")); but it output undefined . But if I add console.log(rdata) above the return line, it logs out the correct response while the code console.log(tdb.search("all", "none", "2")); still returns undefined .

I have no idea why. I've thought of one issue that likely happen is that the process order isn't correct which happened with fs and fixed with promise. But I don't think it's the matter since I return the value inside the callback of post.find() , which than I'm totally stuck. Have no idea where other can go wrong?

Answer 1

Your search function doesn't return anything so it will be undefined by default.

Also find is async. If you remove the callback from exec , then it should return a promise. You can then chain a then instead and return that promise:

search: function(ftype, fval, fkey) {
  var query = {};
  if (ftype != "all") {
    query[ftype] = fval;
  }

  return post
    .find(query)
    .lean()
    .exec()
    .then(res => {
      var keys = Object.keys(res);
      var index = keys.indexOf(fkey);
      var rdata = res[fkey];
      rdata.next = res[keys[index + 1]];
      rdata.prev = res[keys[index - 1]];
      return rdata;
    });
}

...and then you should be able to log the result when the promise resolves using then :

tdb
  .search("all", "none", "2")
  .then(data => console.log(data));

---

Another way you could do this is to pass a callback function to the search function, and then invoke it with the data when ready:

search: function(ftype, fval, fkey, callback) {
  var query = {};
  if (ftype != "all") {
    query[ftype] = fval;
  }

  post.find(query).lean().exec((err, res) => {
    var keys = Object.keys(res);
    var index = keys.indexOf(fkey);
    var rdata = res[fkey];
    rdata.next = res[keys[index + 1]];
    rdata.prev = res[keys[index - 1]];

    callback(rdata);
  });
}

...and then you should be able to log the result from your callback function:

tdb.search("all", "none", "2", (data) => {
  console.log(data);
});

---

I hope this helps.