title: Rails validation on custom action
body: <p>I know you can do:</p>
  
  <pre><code>validates :cat_no, :presence =&gt; true, :on =&gt; :create
  </code></pre>
  
  <p>I have a custom action:</p>
  
  <pre><code> def approve
      @product = Product.find(params[:id])
      @product.update_attribute :approved, true
      redirect_to product_path(@product)
   end
  </code></pre>
  
  <p>And i'd like to do:</p>
  
  <pre><code>validates :cat_no, :presence =&gt; true, :on =&gt; :approve
  </code></pre>
  
  <p>This doesn't seem to work. Is there a simple way of doing this?</p>