public function slice($offset, $length = null)
    {
        $keys = $this->getKeys();

        $keys = array_slice($keys, $offset, $length);

        $collection = $this->getNewCollection();

        foreach ($this as $key => $entity) {
            if (in_array($key, $keys)) {
                $collection[$key] = $entity;
            }
        }

        return $collection;
    }