title: fields_for, formtastic, ActiveMerchant, and validation errors
body: <p>I'm trying to use <a href="http://github.com/justinfrench/formtastic" rel="nofollow noreferrer">Formtastic</a> to create a payment form, since I'd like to use the inline errors. I'm using ActiveMerchant to handle the billing.  I have the following form:</p>
  
  <pre><code>&lt;%= semantic_form_for @payment do %&gt;
    &lt;%= form.inputs do %&gt;
      &lt;%= form.input :email, :label =&gt; "Email Address" %&gt;
  
      &lt;%= form.semantic_fields_for :credit_card_attributes do |cc| %&gt;
        &lt;%= cc.input :number, :label =&gt; "Credit Card Number" %&gt;  
        &lt;%= cc.input :first_name, :label =&gt; "First Name" %&gt;
        &lt;%= cc.input :last_name, :label =&gt; "Last Name" %&gt;
        &lt;%= cc.input :month, :label =&gt; "Expiration Month" %&gt;
        &lt;%= cc.input :year, :label =&gt; "Expiration Year" %&gt;
        &lt;%= cc.input :verification_value, :label =&gt; "Verification Code" %&gt;
      &lt;% end %&gt;
    &lt;% end %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>And this is what's in my <code>Payment</code> model:</p>
  
  <pre><code>class Payment &lt; ActiveRecord::Base
    validates_associated :credit_card, :on =&gt; :create
  
    def credit_card_attributes=(attrs)
      @credit_card = ActiveMerchant::Billing::CreditCard.new(attrs)
    end
  
    def credit_card
      @credit_card
    end
  end
  </code></pre>
  
  <p>When I submit an invalid credit card, it figures out that it's invalid, but I don't get any of the inline errors from formtastic.</p>
  
  <p>I figure there's probably something simple I'm missing here, I'm just not sure what.</p>
  
  <p>This is on Rails 3.</p>