Documents\Article::addTag PHP Method

    public function addTag($tag)
    {
        $this->tags[] = new Tag($tag);
    }

 public function testCollectionField()
 {
     $classMetadata = $this->dm->getClassMetadata('Documents\\Article');
     $persister = new BasicDocumentPersister($this->dm, $classMetadata);
     $this->dm->getUnitOfWork()->setDocumentPersister('Documents\\Article', $persister);
     $article = new Article();
     $article->setTitle('test');
     $article->setBody('test');
     $article->setCreatedAt('1985-09-04 00:00:00');
     $article->addTag('tag 1');
     $article->addTag('tag 2');
     $article->addTag('tag 3');
     $article->addTag('tag 4');
     $this->dm->persist($article);
     $this->dm->getUnitOfWork()->computeChangeSets();
     $update = $persister->prepareUpdateData($article);
     $this->assertTrue(array_key_exists('$pushAll', $update));
     $this->assertTrue(array_key_exists('tags', $update['$pushAll']));
     $this->assertEquals(4, count($update['$pushAll']['tags']));
     $this->assertFalse(array_key_exists('$pullAll', $update));
     $this->dm->flush();
     $this->dm->clear();
     unset($article);
     $article = $this->dm->findOne('Documents\\Article');
     $this->assertEquals(array('tag 1', 'tag 2', 'tag 3', 'tag 4'), $article->getTags());
     $article->removeTag('tag 1');
     $article->removeTag('tag 3');
     $article->addTag('tag 5');
     $this->dm->getUnitOfWork()->computeChangeSets();
     $update = $persister->prepareUpdateData($article);
     $this->assertTrue(array_key_exists('$pushAll', $update));
     $this->assertTrue(array_key_exists('tags', $update['$pushAll']));
     $this->assertEquals(1, count($update['$pushAll']['tags']));
     $this->assertTrue(array_key_exists('$pullAll', $update));
     $this->assertTrue(array_key_exists('tags', $update['$pullAll']));
     $this->assertEquals(2, count($update['$pullAll']['tags']));
     $this->dm->flush();
     $this->dm->clear();
     unset($article);
     $article = $this->dm->findOne('Documents\\Article');
     $this->assertEquals(array('tag 2', 'tag 4', 'tag 5'), $article->getTags());
 }