prevent duplicate for Composite key columns

at last, i added a column brandcategories, in form i hide it (display:none) and assign value using javascript code as 
brand_id + category_id
in Model added a validation "is_unique" for this new column. 
its working fine. its a way around when i can't have a solution without adding this extra column in table.
regards.

Newbie here in CodeIgniter World !! Please appologize if any nonsense from me.  

environment: Windows 10, XAMPP 3.3.0, VS Code, SQLyog Community Ed.