NodeJs猫鼬如何从“ find（）。then”中的“ find（）。then”中获取数据？

Question

Sorry for my Title, I don't know what can I put.

Can you help me please, I would like to print data from a "then" in a "then" ?

Thank you

models.book.find()
  .then( function (content) {
    var i = 0;

    while (content[i]) {
      models.author.findOne({"_id": content[i].author_id}, function(err, data) {
        console.log(data); //here, it' good
        content[i] = data;

        MY_DATA = content;

        return MY_DATA;
      });

      i++;
    };
  })
  .then(function (result) {
    console.log(result); // here I would like to print MY_DATA
  });

Answer 1

There are a number of problems with your code, and I don't think it's behaving as you're expecting it to.

Chaining Promises

In order to effectively chain promises how you're expecting, each promise callback needs to return another promise. Here's an example with yours changed around a bit.

var promise = models.book.find().exec(); // This returns a promise
// Let's hook into the promise returned from 
var promise2 = promise.then( function (books) {
  // Let's only get the author for the first book for simplicity sake
  return models.author.findOne({ "_id": books[0].author_id }).exec(); 
});

promise2.then( function (author) {
  // Do something with the author
});

In your example, you're not returning anything with your callback ( return MY_DATA is returning within the models.author.findOne callback, so nothing happens), so it's not behaving as you're expecting it to.

model.author.findOne is asynchronous

model.author.findOne is asynchronous, so you can't expect to call it multiple times in the callback without handling them asynchronously.

// This code will return an empty array
models.book.find( function (err, books) {
  var i = 0, results = [];

  while (books[i]) {
    models.author.findOne({ "_id": books[i].author_id}, function (err, data) {
      // This will get called long after results is returned
      results.push(data);
    });

    i++;
  };

  return results; // Returns an empty array
});

Handling multiple promises

Mongoose uses mpromise , and I don't see a method to handle multiple promises together, but here's a way your case could be done.

var Promise = require('mpromise');

models.book.find().exec()
  .then( function (books) {
    var i = 0,
        count = 0,
        promise = new Promise(),
        results = [];

    while (books[i]) {
      models.author.findOne({ "_id": books[i].author_id }, function (err, author) {
        results.push(author);
        count++;
        // Keep doing this until you get to the last one
        if (count === books.length) {
          // Fulfill the promise to get to the next then
          return promise.fulfill(results);
        }
        return;
      });
    }

    return promise;
  })
  .then( function (results) {
    // Do something with results
  });

I don't know if this will work exactly like it is, but it should give you an idea of what needs to be done.