What is the secure solution for correcting mass assignment vulnerabilities in Rails 3?

I'm working on securing an existing Rails 3 project. Running brakeman, I'm receiving this warning for many model classes: "Potentially dangerous attribute available for mass assignment: :example_id"

All of the warnings are on _id columns.

The models currently look like this:

class Comment < ActiveRecord::Base
    attr_accessible :commentable_id, :content, :user_id

Along with the main question, if the _id columns are removed to satisfy the warnings, what is the proper way to set these fields then?