title: rails assocations using column other than primary key
body: <p>i am fairly new to rails and want to take advantage of the associations ActiveRecord provides in trying to set up a new project and schema. i have two models, users and questions. when a user is being set up by an administrative user, i have a form with checkboxes for a list of categories. i want these categories to map to specific questions to ask the setup user at a later time. my thinking is i have a mapping table, user_to_categories, that will allow me to get from the user to the necessary questions. but the questions table will have a primary key of question_id, so the has_many :through does not seem to work here. since many questions can be in the same category, category_id will just be another column in the questions table. any suggestions on how to set this up to take advantage of rails and active record associations?</p>