Podařilo se mi uvést dotaz do práce vložením from
sub_query do počátečního příkazu 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");