title: ruby/rails NoMethodError? when using custom validation method
body: <p>i'v been new to ruby and rails and encountered a rather strange error:</p>
  
  <pre><code>class Person &lt; ActiveRecord::Base
      validates_presence_of :description, :if =&gt; require_description_presence?
  
      def require_description_presence?
          self.can_send_email
      end
  end
  </code></pre>
  
  <p>raises </p>
  
  <pre><code>NoMethodError in PeopleController#index
  
  undefined method `require_description_presence?' for #&lt;Class:0x4c4fadc&gt; 
  </code></pre>