public function getModifiedProperties()
    {
        if ($this->read_only) {
            return array();
        }

        $modified_properties = array();
        foreach ($this->getProperties() as $name => $value) {
            $hashed_value = $this->getHashValue($value);
            if (
                array_key_exists($name, $this->property_hashes) &&
                $hashed_value !== $this->property_hashes[$name]
            ) {
                $modified_properties[$name] = $value;
            }
        }

        return $modified_properties;
    }