public function removeSystemFields()
    {
        $properties = $this->fields;
        unset(
            $properties['created'],
            $properties['changed'],
            $properties['changer'],
            $properties['changer_id'],
            $properties['creator'],
            $properties['creator_id']
        );

        foreach ($properties as $key => $field) {
            // remove system fields
            if ('__' == substr($key, 0, 2)) {
                continue;
            }

            $this->properties[$key] = $field->getValue();
        }
    }