Reg: validation for has_many relation table

Hi,    I am facing one problem in validation..I have two tables articles and user both are has_many relation joint table has article_id ,user_id , read_at and rating

In front end there is two check box for read_at and rating

so at the time of saving article i want to validate either one of the check box should be where should place the validation..

please help me

with regards, shiva