added unit test for feature

app/models/photo-album.js

@@ -37,9 +37,9 @@ export default class PhotoAlbum extends Model {
 
   get amountOfTaggedPhotos() {
     var counter = 0;
-    for (var photo of this.photos._objects) {
+    this.photos.content.forEach((photo) => {
       counter += photo.amountOfTags > 0 ? 1 : 0;
-    }
+    })
     return counter;
   }
 

tests/unit/models/photo-album-test.js

@@ -0,0 +1,29 @@
+import { run } from '@ember/runloop';
+import { module, test } from 'qunit';
+import { setupTest } from 'ember-qunit';
+
+let album;
+let photos = [];
+
+module('Unit | Model | photo-album', function (hooks) {
+  setupTest(hooks);
+
+  hooks.beforeEach(function () {
+    const store = this.owner.lookup('service:store');
+    photos = [
+      store.createRecord('Photo'),
+      store.createRecord('Photo'),
+      store.createRecord('Photo'),
+    ];
+    album = store.createRecord('PhotoAlbum', { photos });
+  });
+
+  test('Photo count', function (assert) {
+    assert.expect(2);
+    photos[0].amountOfTags = 0;
+    photos[1].amountOfTags = 1;
+    photos[2].amountOfTags = 5;
+    assert.equal(album.amountOfPhotos, 3, "Amount of photos is correct");
+    assert.equal(album.amountOfTaggedPhotos, 2, "Amount of tags in album is correct");
+  });
+});