New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  store: Ember.inject.service(),
  
  count: 0,
  foo: null,
  
  init() {
    this._super(...arguments);
    
    this.set('foo', this.get('store').createRecord('foo'));
  },

  _manager: Ember.computed('foo.bars.@each.isBlank', function() {
    this.incrementProperty('count');
    
    Ember.run.scheduleOnce('afterRender', this, 'update');
  }),

  update() {
    const
      bars = this.get('foo.bars'),
      lastIndex = bars.get('length') - 1,
      blankBars = bars.filter((bar, index) => {
        return index !== lastIndex && bar.get('isBlank');
      });
    
    blankBars.forEach((blankBar) => {
      // first disassociate it to update the UI
      blankBar.set('foo', null);

      // garbage collect
      Ember.run.later(blankBar, 'rollbackAttributes');
    });
    
    if (!bars.get('lastObject.isBlank')) {
      // always ensure a blank record at the end
      bars.createRecord();
    }
  }
});

models.bar.js

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

export default Model.extend({
  qux: attr('string'),
  
  isBlank: Ember.computed('qux', function() {
    return Ember.isBlank(this.get('qux'));
  }).readOnly()
});

models.foo.js

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

export default Model.extend({
  bars: hasMany({ async: false })
});

templates.application.hbs

{{_manager}}

<div>Count: {{count}}</div>

{{#each foo.bars as |bar|}}
  <div>{{input value=bar.qux}}</div>
{{/each}}

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"
  }
}