title: Using dynamic finders to specify NOT NULL
body: <p>I very often want to use dynamic finders to specify NOT NULL. So…</p>
  
  <p>this works:</p>
  
  <pre><code>Widget.find_all_by_color('blue')
  </code></pre>
  
  <p>this works:</p>
  
  <pre><code>Widget.find_all_by_color(nil)
  </code></pre>
  
  <p>But how can I do</p>
  
  <pre><code>SELECT * FROM `widgets` WHERE `color` IS NOT NULL;
  </code></pre>
  
  <p>?</p>