SELECCIONE de la subconsulta en Codeigniter Active Record

Logré que la consulta funcionara poniendo from sub_consulta en la declaración de selección inicial:

$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");