如何在Rails中生成连接表

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.I have a products model in my rails app, and wi