title: Model.association is returning nil, even with a valid id
body: <p>I have two models, Webcast, and Session. Webcast belongs to Session, and Session has one webcast. Also, I'm using STI with Session, it inherits from the model Schedulable, and has type = 'Session'.</p>
  
  <p>I have a Session record, lets call it session, and a Webcast record, we'll call it webcast.</p>
  
  <p>webcast.session_id is equal to session.id, and vice versa</p>
  
  <p>yet, when I query the association, webcast.session I get nil. however, session.webcast returns the webcast record.</p>
  
  <p>Also, I have tried querying Webcast.session on past Webcast record and it loads the associated record just fine.</p>
  
  <p>Any ideas what could be happening here? Thanks in advance.</p>
  
  <p><strong>Update:</strong></p>
  
  <p>Queries:</p>
  
  <p>(session.webcast)</p>
  
  <pre><code>Webcast Load (1.0ms)  SELECT `webcasts`.* FROM `webcasts` WHERE `webcasts`.`session_id` = 2679 LIMIT 1
  </code></pre>
  
  <p>(webcast.session)</p>
  
  <pre><code>Session Load (0.3ms)  SELECT `schedulables`.* FROM `schedulables` WHERE `schedulables`.`import_id` IS NULL AND `schedulables`.`type` IN ('Session')  
  AND `schedulables`.`id` = 2679 ORDER BY `schedulables`.`starts_at` ASC LIMIT 1
  </code></pre>
  
  <p>Also, I just noticed that if I load up the specific session record that the webcast belongs too, then reload the webcast record and try webcast.session again it will return the session record rather than nil.</p>