I have a problem about to get an object from arraycollection of objects with Id,so I do :

protected $_rootObject;

public function __construct(myClasse $rootObject)
{
    $this->_rootObject= $rootObject;
}

public function getObjectById($id)
{
    $value = null;
    foreach($this->_rootObject as $root)
      {
            if ($id == $root->getId())
            {
                $value = $root;
                break;
            }
       }

    return $value;
}

Then the function return "NULL" so it's dosn't work...