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 ....so 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 checked.so where should place the validation..

please help me

with regards,
shiva