title: Why do I get "Unknown validator: 'MessageValidator'"?
body: <p>I get this error</p>
  
  <blockquote>
    <p>Unknown validator: 'MessageValidator'</p>
  </blockquote>
  
  <p>I have no idea why I'm getting that.<br>
  What's wrong with my code?</p>
  
  <pre><code>validates :title,   
      :presence =&gt; true,   
      :uniqueness =&gt; true,   
      :length =&gt; { :maximum =&gt; 100 },
      :message =&gt; "Must be input and has to be less than 100 characters, and unique."
  </code></pre>