Saving a Many-to-Many Relationship in Ember JS - Firebase

I'm trying to understand how to save a many-to-many relationship in Ember. And for this purpose, I've set up a JS Bin with a Firebase backend to play around in.

In this example, one blog post can have many authors. And one author can create many blog posts. Here are the models:

 App.Post = DS.Model.extend({
  msg: DS.attr(),
  authors: DS.hasMany('user', { async: true }), 
});

App.User = DS.Model.extend({
  name: DS.attr(),
  posts: DS.hasMany('post', { async: true })
});

Here, I'm simply playing around with hard-coded data. I want to save a blog post and two authors. Then I want to add the associated 'post' record to the users' table.

So, here's how my non-working, 'savePost; function looks:

savePost: function(){
      var post = this.store.createRecord('post',{msg: 'round the rugged rock'});
      var author1 = this.store.createRecord('user', {name: 'john'});
      var author2 = this.store.createRecord('user', {name: 'jane'});
          post.save()
//please ignore the fact that author saving happens inside saving a post 
//I just needed it to demonstrate what I want to do

          .then(function(){   
            author1.save();})
          .then(function(){
            author2.save();})
          .then(function(){
         post.get('authors').addObjects(author1, author2)
          .then(function(){
           post.save();
         });
      });
    }

Am I supposed to be using the Ember Embedded Records mixin for this?

If so, could you please show the usage and syntax for this?