title: Rails left join on two keys isn't returning joined fields
body: <p>I have two tables events and assignments</p>
  
  <pre><code>events                       assignments
  =====                        ============
  sport_id                     sport_id
  home_school_id               school_id
  name                         division_id
  </code></pre>
  
  <p>And I'm trying to select the events and join the assignments like so:</p>
  
  <pre><code>joins = "LEFT JOIN assignments sa on events.sport_id = sa.sport_id 
           AND events.home_school_id = sa.school_id"
  events = Event.find(:all, :joins =&gt; joins)
  </code></pre>
  
  <p>The problem is this doesn't return the division _id. Why not ? Is it because there isn't an association ?</p>
  
  <p>Shouldn't I be able to do <code>events.first.division_id</code> ? I've tried this in mysql and it works fine.</p>
  
  <h1>edit</h1>
  
  <p>My Event model association</p>
  
  <pre><code>has_many :assignments,
    :primary_key =&gt; :sport_id,
    :foreign_key =&gt; :sport_id,
    :finder_sql =&gt;
        'SELECT sa.* ' +
        'FROM events e, assignments sa ' +
        'WHERE e.sport_id = sa.sport_id AND e.home_school_id = sa.school_id ' +
        'ORDER BY e.event_date'
  </code></pre>