title: Validate Presence Of At Least One of Several
body: <p>I have an ActiveRecord model with the following three attributes:</p>
  
  <pre><code>data_1
  data_2
  data_3
  </code></pre>
  
  <p>How do I write a validation that checks whether at least one of those fields is <em>not</em> <code>blank?</code></p>