title: NoMethod error when adding new column in admin/products/_form
body: <p>I think I followed the <a href="https://guides.solidus.io/developers/customizations/customizing-attributes" rel="nofollow noreferrer">guide</a> on adding a new column to the database:</p>
  <ul>
  <li>Created &amp; ran a migration for the spree_prices table for new columns &quot;msrp&quot; and &quot;msrp_currency&quot;</li>
  <li>Added to spree initializer with
  <code>Spree::PermittedAttributes.product_attributes &lt;&lt; [:msrp, :msrp_currency]</code></li>
  <li>I tried &quot;price_attributes&quot; but server won't start saying it doesn't exist.</li>
  <li>Added this to a views/spree/admin/products/_form.erb.html file:
  <pre><code>&lt;%= f.field_container :msrp do %&gt;
    &lt;%= f.label :msrp, 'MSRP' %&gt;
    &lt;%= render &quot;spree/admin/shared/number_with_currency&quot;, 
             f: f, 
             amount_attr: :msrp, 
             currency_attr: :msrp_currency %&gt;
    &lt;%= f.error_message_on :msrp %&gt;
    &lt;%= f.error_message_on :msrp_currency %&gt;
  &lt;% end %&gt;
  </code></pre>
  </li>
  </ul>
  <p>When I visit Product's create/edit form, it errors with ActionView::Template::Error (undefined method `msrp' ....</p>