title: Rails: belongs_to association not returning the correct object
body: <p>I've got an Order model with two belongs_to associations, each to a different sub-class of my Account model.  After loading an order from the database, both associations point to the same model, even though the foreign keys are correct. </p>
  
  <pre><code>class Account &lt; AR::Base
  end
  
  class FooAccount &lt; Account
  end
  
  class BarAccount &lt; Account
  end
  
  class Order &lt; AR::Base
    belongs_to :account, :class_name =&gt; 'FooAccount', 
      :foreign_key =&gt; :account_id
    belongs_to :different_account, :class_name =&gt; 'BarAccount', 
      :foreign_key =&gt; :different_account_id
  end
  </code></pre>
  
  <p>Console does something like this:</p>
  
  <pre><code>o = Order.find(42)
  =&gt; #&lt;Order id: 42, account_id: 11, different_account_id: 99&gt;
  a = Account.find(11)
  =&gt; #&lt;FooAccount id: 11, type: "FooAccount"&gt;
  d = Account.id(99)
  =&gt; #&lt;BarAccount id: 99, type: "BarAccount"&gt;
  o.account_id
  =&gt; 11
  o.account
  =&gt; #&lt;BarAccount id: 99, type: "BarAccount"&gt;
  o.different_account_id
  =&gt; 99
  o.different_account
  =&gt; #&lt;BarAccount id: 99, type: "BarAccount"&gt;
  </code></pre>
  
  <p>The foreign key values are correct, but the object referenced by the association is not!  What am I doing wrong?</p>