title: Rails foreign key other than id
body: <p>I have an ActiveRecord named Ad, and it has column id and <code>server_id</code>. They are both unique. id is given by Rails, but what's meaningful here is <code>server_id</code>.
  <br>
  Then I need to create another <code>ActiveRecord</code> named Bid which has 1 to 1 relationship with <code>Ad</code>. Bid has a key <code>ad_id</code> which refers to Ad.<code>server_id</code>.<br></p>
  
  <p>I know that I should specify <code>Ad has_one Bid</code> and <code>Bid belongs_to Ad</code>, and specify foreign key name through foreign_key: <code>"ad_id"</code>. What's troubling me here is I can only find out how to let Bid.<code>ad_id</code> refer to Ad.id while I want it refers to Ad.<code>server_id</code>.<br>
  Can anybody show me how to achieve this? Thanks!</p>