title: How to group collection by columns with rails
body: <p>My table named stocks contains <code>product_id, color_id, storage_id and in_stock</code> as columns. </p>
  
  <p>For a given product I want to group all stocks by storage, then for each storage I want to show product (color): in_stock</p>
  
  <p>How should I write a method and how to render?</p>