How to access relations before create in model using octobercms?

Remember, first of all, this is related to octobercms (laravel) - http://octobercms.com

I am doing something before creating an record.. My crud is already working with relations. My structure is this: Coupom coupom_categories Categories

I want list all those categories selected in the crud to make something with then before create coupom..

Any help?

via Chebli Mohamed