Node.js: Creating multiple MongoDB docs over iterated JSON POST data

A webpage sends JSON data via POST to my Node.js App (MEAN-environment using Mongoose). The JSON file looks like this (excerpt):

   Firstname: 'XY',
   Surname: 'asd', 
   Articles: 
       [ { title: '1', description: 'XY' },
         { title: '2', description: 'XY' },
         { title: '3', description: 'XY' } 

The purpose is to create an Author in a mongodb database, add the author to an additional directory, store associated articles and create references to those articles in the author document (code excluded). This is the code that handles the request:

[...]
    async.waterfall([
            //Create random code for author (besides mongodb-specific id)
            function(callback){
                newAuthorCode.randomAuthorCode(function(err, code) {
                    callback(null, code);    
                });
            },

            //Save new author to db
            function(code, callback){
                var newAuthor = new Author({firstname: req.body.Firstname,
                                        surname: req.body.Surname,
                                        identcode: code
                });

                newAuthor.save(function (err){
                    callback(null, newAuthor);
                });
            },

            //Add new author to an additional directory
            function(newAuthor, callback){
                Directory.update({_id: req.user._id}, {$push: {authorids: newAuthor._id }}, function(err, update){
                    if (update){
                        callback(null);
                    }
                });
            },

            //saves articles to db
            function(callback){
                var keys = Object.keys(req.body.articles);

                for(var i=0, length=keys.length; i<length; i++){
                    var newArticle = new Article({title: req.body.Articles[keys[i]].title,
                                               description: req.body.Articles[keys[i]].description

                    });

                    newArticle.save(function (err){
                        console.log(newArticle._id); // <--- !!!!!!!
                    });
                }
                callback(null);
            }

        ], function (err, result) {
            console.log('DONE!!!');
            res.send('200');
        }); 

My problems:

1) The marked line of code where I try to output all IDs of the generated articles only delivers i-times the ID of the last article stored (in this case of article 3).

2) Problem 1 leads to the issue that I can not create references of the newly created articles in the author document (stored in a different collection) as I can't access no article IDs but the last one!?

3) Sometimes the author, as well as the articles, are created multiple times in the database (with huge time gap in between)!?

Thanks for any advice as I am running out of ideas.

Igor