Udało mi się uruchomić zapytanie, umieszczając from
sub_query do początkowej instrukcji select :
$this->db->select("*,
(select image_path from image where image_table = 'model' and image_table_id = model_id
group by image_table_id limit 1) as ModelImg FROM
(SELECT * FROM $view where offer_table = 'derivative' order by offer_order) x");