http://stackoverflow.com/questions/42964727/how-can-i-pass-ids-for-hasmany-relationship-using-store-createrecord/43031189

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

models.customer.js

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

export default Model.extend({
  products: hasMany('product')
});

models.product.js

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

export default Model.extend({
  customer: belongsTo('customer')  
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model(){ 
    let product1 = this.store.createRecord('product',{customer:1});
    let product2 = this.store.createRecord('product',{customer:2});    
    return this.get('store').createRecord('customer', {products:[product1,product2]}); 
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
{{log model}}
<br>
{{outlet}}
<br>
<br>

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"
  }
}