Associating has_many relationships in Rails 3 using checkboxes

(Not checkboxes, in my case, but a checkbox example is easy, and more common.)

Originally I thought I needed accepts_nested_attributes_for, but that seems to be mostly for when we're creating the related objects, which I'm not--I need to save relationships to existing objects.

My example (github) uses a simple product/category relationship. We need to get a product's categories, we need to get