What is the best updating way for ODM mongo collection model?

When I try to update an entry validation fires an uniqueness error. I want to get my object by label attribute and update other nodes of object.

Here is the update code:

    $shelf = Collections\ShelfCollection::findFirst(array(
        array('label' => 'my-label')
    ));

    if ($shelf) {
            $shelf->value = 'changed-value';
            if ($shelf->save() == false) {
                foreach ($shelf->getMessages() as $message) {
                    echo $message, "\n";
                }
            } else {
                echo "Shelf value updated successfully!<br />";
            }
    }
    else {
        echo 'Shelf label not found';
    }

Simply you can take a look to my collection model:

<?php
namespace Collections;

use Validator\Collection\UniqueValidator As Uniqueness,
    Phalcon\Mvc\Model\Message as Message;

class ShelfCollection extends \Phalcon\Mvc\Collection {

    public $label;
    public $value;

    public function getSource()
    {
        return "shelf";
    }

    public function initialize()
    {
        // Auto generate _id keys
        $this->useImplicitObjectIds(true);
    }

    public function validation()
    {
        $this->validate(new Uniqueness(
            array(
                "field"   ="value",
                "message" ="Value must be unique"
            )
        ));

        $this->validate(new Uniqueness(
            array(
                "field"   ="label",
                "message" ="Label must be unique"
            )
        ));

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

    public function beforeSave()
    {
        // May Apply Here Business Layout Model
    }
}