How to validate unique field in mongodb

Hello. How i can validate mongodb collection by Uniqueness? I get error

Call to undefined method ::getschema()

my code is

use Phalcon\Mvc\Model\Validator\Uniqueness;
use Phalcon\Mvc\Model\Validator\PresenceOf;
use Phalcon\Mvc\MongoCollection;

class Users extends MongoCollection {

    public $name;
    public $password;

    public function validation() {
        $this->validate(new PresenceOf([
            "field" => "name",
            "message" => "The name is required",
        ]));
        $this->validate(new PresenceOf([
            "field" => "password",
            "message" => "The pasword is required",
        ]));
        $this->validate(new Uniqueness([
            "field" => "name",
            "message" => "Name must be unique",
        ]));

        return $this->validationHasFailed() !== true;
    }

}