how to grab one side of many to many entities in Cake 3

index_by_tag.php

<?php

// I have this in BookmarksController 
// I am following the bookmarkr tutorial in http://book.cakephp.org/3.0/en/tutorials-and-examples/bookmarks/intro.html


/**
 * Index view by tag method
 *
 * @return void
 */
	public function index_by_tag($tag_id) {
		$conditions = [
			'Bookmarks.public' => true,
		];
		$this->paginate = [
			'conditions' => $conditions,
			'contain' => [
			    'Users' => function ($q) {
			       return $q
			            ->select(['username', 'id']);
			    }
			],
			'order' => [
				'Bookmarks.updated' => 'desc', 
				'Bookmarks.id' => 'desc'
			]
	    ];
	    $this->set('bookmarks', $this->paginate($this->Bookmarks));
	}