title: How to migrate has_and_belongs_to_many to has_many through?
body: <p>We have a simple has_and_belongs_to_many relation between two models. We would like to add in some paramaters to that model, so we need to change it to a has_many :through sort of model. </p>
  
  <p>As I know it, we need to add in an id column (along with whatever columns we want additionally). However, I'm not clear 100% on how to do this. If we add an integer column :id, will rails know that that is the 'id' primary key?</p>
  
  <p>We're using the latest 3.x. </p>