title: Return default record if no other related record exists
body: <p>Say I have a table defined such that:  </p>
  
  <pre><code>Entities has_many Images  
  Images has_one Entity
  </code></pre>
  
  <p>I have a record in the Images table that has no value for entity_id that I would like to be the default image for any new entity.  </p>
  
  <p>When I use:  </p>
  
  <pre><code>@entities.images.find(:all, :conditions =&gt; ['image_type = ?', 'avatar])
  </code></pre>
  
  <p>I would like the default record from the Images table to be returned iff there is no image for the current entity</p>
  
  <p>Is this possible, and if so, how?</p>
  
  <p>Thanks.</p>