title: How to change primary key in rails migration file?
body: <p>I need to migrate an old mysql table like this:</p>
  
  <pre><code>Products
    name (string, primary_key)
  </code></pre>
  
  <p>to this schema:</p>
  
  <pre><code>Products
    id (integer, primary_key, auto_generated)
    name (unique)
  </code></pre>
  
  <p>I need the Products.id values populated in the new table.
  How can i write the rails migration file? I am using Rails 3.2.7</p>
  
  <p>I have 2 problems now:
  1. I can't find a method to remove primary key in ActiveRecord::Migration
  2. I don't know how to generate values for newly added primary key.</p>