How to use promise to avoid callback hell? [duplicate]

Question

So I have a Posts collection

{
  id: String,
  comments: [String], # id of Comments
  links: [String], #id of Links
}

Comments: { id: String, comment: String, }

Links: { id: String, link: String, }

Find a post with comments and links belong to it by id:

Posts.findOne({id: id}, function(post) {
  Comments.find({id: post.id}, function(comments) {
    Links.find({id: post.id}, function(links) {
      res.json({post: post, comments: comment, links: links})
    })
  })
})

How to use Promise(http://mongoosejs.com/docs/promises.html) to avoid callback hell?

var query = Posts.findOne({id: id});
var promise = query.exec();

promise.then(function (post) {
  var query1 = Comments.find({id: post.id});
  var promise1 = query1.exec();
  promise1.then(function(comments) {
    var query2 = Links.find({id: post.id});
    var promise2 = query2.exec();
    promise2.then(function(links) {
      res.json({post: post, comments: comment, links: links})
    })
  })
});

Seems no good......

Answer 1

You are nesting the callbacks. You don't need to do this. If you return a promise from .then then any .then you chain to it will be resolved when that promise gets resolved:

promise.then(post => Comments.find({id: post.id})
  .then(comments => Links.find({id: post.id})
  .then(links => {});

The comments query does not depend on links so you can actually do both queries at once:

promise.then(post => {
  return Promise.all([
     post,
     Comments.find({id: post.id}),
     Links.find({id: post.id}),
  ]);
}).then(data => res.json({
  post: data[0],
  comments: data[1],
  links: data[2],
});

If you use a library like bluebird you can also use something like the spread operator to make the names more transparent.

---

I would also look into using co for generator-based control flow as I think this is even clearer:

co(function* () {
  const post = yield Posts.findOne({id});
  const [comments, links] = yield [
    Comments.find({id: post.id}),
    Links.find({id: post.id}),
  ];

  res.json({post, comments, links});
});

Answer 2

Try whit this:

function getPost(id) {
  return Post
    .findOne({id: id})
    .then( post => {
      return post;
    });
}

using Q module

function getCommentsAndLinks(post) {
  return Q.all([
    Comment.find({id: post.id}),
    Links.find({id: post.id})
  ])
  .done( results => {
    let comments = results[0];
    let links = results[1];
    return [post, comments, links];
  })
  .catch( err => {
  // handle err
  })

on controller

getPost(postId)
.then(getCommentsAndLinks)
.then( results => {
  let post = results[0];
  let comments = results[1];
  let links = results[2];
  // more code here
})
.catch( err => {
// handle err
}) 

but i suggest you not save string of IDS, save the instance of object, so you can use populate to get all data of comments and links, something like this:

Post
.findOne({id: id})
.populate('comments')
.populate('links')
.then( post => {
    // here have the post with data of comments and links
});