title: How to generate a Join table in Rails
body: <p>I have a products model in my rails app, and with in the products table is a categories column, which is comma separated. I want to create a join table (product_categories) so that the categories are no longer comma separated (making it easier to count etc). Im am planning on joining by id on the product table and have id, product_id and categories on the join table.</p>
  
  <p>Is it best practice to run a migration to create a join table or should I generate a model then run the migration to create the join table?</p>
  
  <p>I am running rails 3.1.0. </p>