title: validates_presence_of if condition on rails 3.2 and mongoid + simple_form
body: <p>I want validate presence of these 2 attributes <code>:shipping_cost</code> and <code>:shipping_cost_anywhere</code> if the attribute <code>:shipping</code> is equal to <code>true</code>. and If </p>
  
  <p>I have this in my model but not working fine for me:</p>
  
  <pre><code>validates_presence_of :shipping_cost, :shipping_cost_anywhere, :allow_blank =&gt; "true" if :shipping == "true"
  </code></pre>
  
  <p>this is my :shipping attribute:</p>
  
  <pre><code>field :shipping, :type =&gt; Boolean, :default =&gt; "false"
  </code></pre>
  
  <p>How can I do it?</p>
  
  <p>Thank you!</p>
  
  <p>Edited.</p>
  
  <p><strong>I'm using mongoid and simple_form gems</strong></p>