How to keep async, many-to-many relationships up to date using EmberData

Given the following models:

var post = DS.Model.extend({
    tags: DS.hasMany('tag', {async: true, inverse: 'posts'})
});

var tag = DS.Model.extend({
    posts: DS.hasMany('post', {async: true, inverse: 'tags'})
});

Adding or removing a tag looks like this:

var post = this.get('model');

this.store.createRecord('tag', {}).save().then(function (tag) {
    post.get('tags').then(function (tags) {
        tags.addObject(tag);

        post.save().then(function () {
            tag.get('posts').then(function (posts) {
                posts.addObject(post);

                tag.save().catch(function (res) {
                    // handle errors
                });
            });
        }).catch(function (res) {
            // handle errors
        });
    });
});

I feel like there must be a better way of handling the thenables - but I am still trying to wrap my head around promises and how to use them properly.

So, what is the best way to add or remove a tag from a particular post?