MM relation mapping in contao

Haii,

I am a beginner in Contao ,
Currently I am extending the news module inorder to add the related news option for each news .I am able to list all the news by using the below configuration and able to save it to tl_news database as blob.
DCA Configuration

Code:

---

$GLOBALS['TL_DCA']['tl_news']['fields']['related'] = array
        (
        'label' => 'related',
        'exclude' => true,
        'filter' => true,
        'inputType' => 'select',
        'foreignKey' => 'tl_news.headline',
        'eval' => array('multiple' => true, 'fieldType' => 'checkbox', 'foreignTable' => 'tl_news', 'titleField' => 'headline', 'searchField' => 'headline', 'managerHref' => 'do=news&table=tl_news'),
        'sql' => "blob NULL",
);

---

For mapping the related news i have created tl_news_related table in database which contains two fields news_id(local) and related_id(foreign).
But I am unable to update relation in the tl_news_related table .Please help me to solve this .