model contains test

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  johnIncluded: Ember.computed('models.[]', 'johnModel', function() {
    return this.get('models').contains(this.get('johnModel'));
  }),
  
  init() {
    this._super(...arguments);
    const johnModel = this.store.createRecord('test-model', { name: 'John' });
    this.set('johnModel', johnModel);
    const models = [
      johnModel,
      this.store.createRecord('test-model', { name: 'Jane' }),
      this.store.createRecord('test-model', { name: 'Chuck' }),
    ];
    
    this.set('models', models);
  },
  
  actions: {
    removeJohn() {
      const newModels = this.get('models')
        .filter(item => item !== this.get('johnModel'));
      this.set('models', newModels);
    },
  }
});

models.test-model.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'),
});

templates.application.hbs

<h1>Welcome!</h1>
<br>
<br>
{{#if johnIncluded}}
  Hurray John Included!
{{else}}
  No John, sorry :(
{{/if}}

<button {{action 'removeJohn'}}>Remove him!</button>
<br>
<br>

twiddle.json

{
  "version": "0.14.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "2.18.2",
    "ember-template-compiler": "2.18.2",
    "ember-testing": "2.18.2"
  },
  "addons": {
    "ember-data": "2.18.2"
  }
}