How to make embedded hasMany relationships work with ember data

I can't get embedded hasMany to work correctly with ember data.

I have something like this

App.Post = DS.Model.extend({
  comments: DS.hasMany('App.Comment')
});

App.Comment = DS.Model.extend({
  post: DS.hasMany('App.Post'),
  name: attr('string')
});

And my API returns the following for GET /post:

[
  {
   id: 1
   comments: [{name: 'test'}, {name: 'test2'}]
  },
  ...
]

I need to send this with POST /post:

[
  {
    comments: [{name: 'test'}, {name: 'test2'}]
  },
  ...
]

I want to work with Ember models and have them make the appropriate requests:

var post = App.store.createRecord(App.Post, hash_post_without_comments);
post.get('comments').createRecord(hash_comment);

App.store.commit(); // This should call the POST api 

and

var posts = App.store.find(App.Post); // This should call the GET api 

When I try something like post: DS.hasMany('App.Post', {embedded: true}), the GET is working but the POST is trying to make a POST for the two records not only the parent one.

EDIT : My Real use case

1- I've just built ember data from master

2- My adapter: RESTAdapter

3- The serializer: JSONSerializer

4- I added

App.MyAdapter.map('App.Join', {
    columns: { embedded: 'always' }
});

5- My Models are:

App.Join = DS.Model.extend({
    rowCount: DS.attr('number'),
    columns: DS.hasMany('App.JoinColumn'),
});

App.JoinColumn = DS.Model.extend({
    join: DS.belongsTo('App.Join')
});

6- When:

var a = App.Join.find(1);
a.get('columns').createRecord({});
App.store.commit();

a POST for joincolumn is sent and the parent is not dirty

What am i missing?