title: "You need to supply at least one validation" but Rails validation exists
body: <p>I have the following validation: </p>
  
  <pre><code>class Event &lt; ActiveRecord::Base
    attr_accessible :starts, :ends
  
    validates :no_multi_day_events
  
    private
      def no_multi_day_events
        if (ends.day != starts.day)
          errors.add(:ends, "No multi-day events")
        end
      end
  end
  </code></pre>
  
  <p>However, when I try to load a page with this text, I get an error: <code>You need to supply at least one validation</code></p>
  
  <p>How do I supply the validation?</p>