title: Add association to existing model using a foreign key
body: <p>Say you have these two models</p>
  
  <pre><code>rails generate model User
  rails generate model Car
  </code></pre>
  
  <p>Now I want to add an associations so that the models acquire the form</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    has_many :cars
  en
  class Car &lt; ActiveRecord::Base
    belongs_to :driver, foreign_key: "driver_id", class_name: "User"
  end
  </code></pre>
  
  <p>What will my migrations look like to add the proper column to car? Should it be a column named driver_id or user_id?</p>
  
  <p>This is a variation on <a href="https://stackoverflow.com/questions/15385087/add-associations-to-exisiting-models">this question</a>.</p>