How to use Database query builder

Here is example of Query builder usage in application's service:

class PostFactory
{
	/**
	 * @var \database\Query
	 */
	private $query;

	function __construct(\database\DB $db)
	{
		$this->query = $db->createQuery();
		$this->query->select("p,*")
			->from("posts p")
			->join("INNER JOIN users u ON u.user_id = p.user_id");
	}

	function getPosts($args = array())
	{
		$args = array_merge(array(
			"posts_per_page" => 10,
			"is_active" => 1
		), $args);

		$this->query->limit($args["posts_per_page"]);
		$this->query->where("p.is_active = ?", $args["is_active"]);

		return $this->query->execute()->fetchCollection();
	}

	function getPostsByUserId($user_id, $args = array())
	{
		$this->query->where("p.user_id = ?", $user_id);
		return $this->getPosts($args);
	}
}