title: ActiveRecord :include not working for belongs_to with foreign_key
body: <p>I have a two models set up like this:</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    # external_id column in database
  end
  
  class UserUpload &lt; ActiveRecord::Base
    belongs_to :user, :primary_key =&gt; "external_id", :foreign_key =&gt; "external_user_id"
  end
  </code></pre>
  
  <p>However, whenever I do</p>
  
  <pre><code>upload = UserUpload.find(id, :include =&gt; :user)
  </code></pre>
  
  <p>The sql that gets emitted is always looking for ID (and using zeros):</p>
  
  <pre><code>select * from users where id in (0,0,0,0 ... 0,0)
  </code></pre>
  
  <p>Am I doing something wrong, or is there a problem using <code>foreign_keys</code> in a <code>belongs_to</code> relationship with <code>include</code>?</p>