Promise nested deep map

Question

i am trying to use Promise to make something easy, but it appear to be a real nightmare with promises. I think i am missing something with it.

I would Like to :

1. Fetch some articles in database
2. Look into each article found and iterate over article.authors Array.
3. Fetch each author in dataBase (including author.images) for each article
4. Send back to client the articles List from step One but updated with article.authors.images

I tryed severals ways using Map / Each / spread / Reduce / _.clone / _cloneDeep

But nothing works as expected

Any help would be appreciate

return Promise.bind({})
        .then(function find_article(){
            return Article.find().sort(req.params.sort).skip(req.params.page).limit(req.params.limit).populateAll()
        }).then(function(articles){
            dataBack = articles
            var articlesPromise =  Promise.map(articles,function(article){
                console.log('------------');
                var AuthorsPromise = Promise.map(article.authors,function(author){
                    return User.findOne(author.id).populateAll().then(function(){
                    })
                })
                return Promise.all(AuthorsPromise).then(function(data){
                    console.log('*******************************');
                    console.log(data);
                    return data
                })

            })
            return Promise.all(articlesPromise).then(function(allArticles){
                    console.log('++++++++++++++++++++++++++++++');

                console.log(allArticles);
            })
        })
        .then(function(WhatIsInThere){
            console.log('somethinAfter');
            console.log(WhatIsInThere);
        })

I got Something like this, but is still doesnt work i am still missing the point of the .all()

Answer 1

This is not trivial task, you need to chain promises and most probably use function like Promise.all() or jQuery.when()

Your code should look like this

// function that fetch from database and returns promise
function getArticleFromDatabase(articleId) {
    return new Promise();
}

// function that fetch from database and returns promise
function getAuthorFromDatabase(authorId) {
    return new Promise();
}

var articleIds = [1, 2, 3]; // lets have some array with article ids
// then turn it into array of promises
var articlePromises = articleIds.map(function(articleId) {
    var articlePromise = getArticleFromDatabase(articleId);
    // return the promise
    return articlePromise.then(function(articleData) {
        // here we have available complete article data
        var articleAuthors = articleData.authors; // supose it's array of author ids
        // lets turn it into author data promises
        var authorsPromises = articleAuthors.map(function(author) {
            return getAuthorFromDatabase(author.id);
        });

        // return new Promise, so our first promise of article data
        // will return promise of article data with authors data
        return Promise.all(authorsPromises)
            .then(function(fullfilledAuthorsData) {
                // fill in authors data
                articleData.authors = fullfilledAuthorsData;
                // return complete article data with authors data
                return articleData;
            });
    });
});

Promise.all(articlePromises).then(function(fullfilledArticleData) {
    // here you have available complete article data from all articles
    // fullfilledActicledata is array mapped from initial array of ids
    // so fullfilledActicleData[0] has data for articleIds[0],
    // fullfilledActicleData[1] has data for articleIds[1] etc.
    // You can use the fullfilledArticleData freely.
});

---

Based on your code

// this method obviously returns Promise already
var articlesPromise = Article
    .find()
    .sort(req.params.sort)
    .skip(req.params.page)
    .limit(req.params.limit)
    .populateAll();

// attach callback via .then()
articlesPromise
    .then(function(articles) {   
        // here we have fullfilled articles data already
        var articlesWithAuthorsPromises = articles.map(function(article) {
            var authorsPromises = article.authors.map(function(author) {
                return User.findOne(author.id).populateAll();
            });

            return Promise.all(authorsPromises)
                  .then(function(fullfilledAuthors) {
                      article.authors = fullfilledAuthors;
                      return article;
                   })
        })

        // return new Promise
        return Promise.all(articlesWithAuthorsPromises)
    })
    // attach another callback via .then()
   .then(function(fullData) {
        console.log(fullData);
    })
    // you should also listen for errors
    .catch(errorCallback)