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