Problem with Subqueries

Hello, 
I can't transform my sql request into codeigniter,
if someone will help me.
Thank you

SELECT a.id_article, a.title, a.text
FROM articles a
JOIN category c ON a.id_category  = c.id_category 
WHERE a.id_article = (
    SELECT MIN(a2.id_article)
    FROM articles a2
    WHERE a2.id_category  = c.id_category 
    AND c.id_parent = 4
)
GROUP BY c.id_category 
ORDER BY a.id_article ASC