title: Rails Custom Validation error message
body: <p>Without getting into all the javascript for a textfield, I'd like the following validation:</p>
  
  <pre><code>validates_length_of :brief_description,
    :maximum=&gt;250, 
    :message =&gt; "Brief is #{self.brief_description.length} long (max is 250)"
  </code></pre>
  
  <p>But the self.brief_description reference in the message doesn't work. What's the best way to do this?</p>
  
  <p>I tried 
      #{params[:brief_description].length}
  as well, but the model doesn't know anything about params...</p>