Loop with asynchronous callbacks in mongoose/mongodb/node - node.js

Question

I am new to nodejs/mongo/mongoose and i am trying to do a very simple thing. I have the following schemas:
var authorSchema = mongoose.Schema({
name: String,
});
Author = mongoose.model('Author', authorSchema);
var bookSchema = mongoose.Schema({
title: String,
isbn: String,
pages: Number,
author: { type : mongoose.Schema.ObjectId, ref : 'Author', index: true }
});
Book = mongoose.model('Book', bookSchema);
I want to create a list of authors with id, name and book count for each author. I have something like this:
exports.author_list = function(req, res){
Author.find({}, function (err, authors){
var author_array = Array();
for (var i=0;i<authors.length;i++){
var author_obj = new Object();
author_obj.id = authors[i]._id;
author_obj.name = authors[i].name;
author_obj.count = 0; //here is the problem
author_array[i] = author_obj;
}
res.writeHead(200, { 'Content-Type': 'application/json' });
res.write(JSON.stringify({ authors: author_array }));
res.end();
});
}
I know how to do a query for the count. My problem is how to do a loop of the authors and populate the output with asynchronous callbacks. What is the proper way to implement this in nodejs fashion?
Thanks

Answer 1

I think you'd want to use something like async to coordinate those requests; map() seems to be a good choice:
Author.find({}, function (err, authors) {
async.map(authors, function(author, done) {
Book.count({author: author._id}, function(err, count) {
if (err)
done(err);
else
{
done(null, {
id : author._id,
name : author.name,
count : count
});
}
});
}, function(err, author_array) {
if (err)
{
// handle error
}
else
{
/*
res.writeHead(200, { 'Content-Type': 'application/json' });
res.write(JSON.stringify({ authors: author_array }));
res.end();
*/
// Shorter:
res.json(author_array);
}
});
});