Add support for rolling back hasMany relationships

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  users: [],
  actions: {
    addUser: function() {
      let user = this.store.createRecord('user', {
        name: 'test'
      });
      
      this.get('users').pushObject(user);
    },
    addPost: function(user) {
      this.store.createRecord('post', {
        name: 'test',
        user
      });
    },
    changePost: function(user, post) {
      let users = this.get('users'),      
      index = users.indexOf(user);
      
      if(index < users.get('length')) {
      	let nextUser = users.objectAt(index + 1);
        
        post.set('user', nextUser);
      } else {
        post.set('user', null);
      }
    }
  }
});

models.post.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
  name: attr('string'),
  user: belongsTo("user")
});

models.user.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
  name: attr('string'),
  posts: hasMany("post")
});

templates.application.hbs

<h1>Welcome</h1>

<a href="javascript:void(0)" {{action 'addUser'}}>
Add User
</a>

<ul>
{{#each users as |user|}}
<li>{{user.name}}

<a href="javascript:void(0)" {{action 'addPost' user}}>
Add Post ({{user.posts.length}})
</a>

<ul>
{{#each user.posts as |post|}}
<li>{{post.name}}

<a href="javascript:void(0)" {{action 'changePost' user post}}>
Push to below User
</a>

</li>
{{/each}}
</ul>
</li>
{{/each}}
</ul>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}