title: Zend SQL using string in join condition
body: <p>I want to have a join like this: </p>
  
  <pre><code>$sql = new Sql($this-&gt;adapter);
  $select = $sql-&gt;select();
  $select-&gt;from(array('E' =&gt; 'Emails'));
  $emailField = 
  $select-&gt;join(array('F' =&gt; 'EntityFiles'), "F.RelatedEntityId = E.EmailId AND F.RelatedEntityType = 'email'", array('FileName' =&gt; 'Name'), $select::JOIN_LEFT);
  </code></pre>
  
  <p>But when I try to run it I get an error saying Unknown column '"<code>email</code>"' in 'on clause'.
  How can I get it to work?</p>