Has_and_belongs_to_many with default

i have used a relaction has_and_belongs_to_many to implementing a
backoffice where the customer settings categories and products: the
products can have multiple categories, it selet them with a combobox.
Now the problem is the customer ask me needly to choice a category by
default: products are indicizated on google, when this links are opening
the product must by reporting the default category.
how i obtain this with an has_and_belongs_to_many relaction?