title: Formtastic nested form field not building has_one association?
body: <p>Given a User who can possibly be an Artist:</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    has_one :artist
  end
  </code></pre>
  
  <p>I've got a User &amp; Artist nested form (using Formtastic gem):</p>
  
  <pre><code>&lt;h1&gt;Artist registration&lt;/h1&gt;
  
  &lt;% @user.build_artist unless @user.artist %&gt;
  
  &lt;%= semantic_form_for @user, :url =&gt; create_artist_path do |f| %&gt;
    &lt;%= f.inputs :username %&gt;
  
    &lt;%= f.semantic_fields_for :artist do |a| %&gt;
      &lt;%= a.input :bio %&gt;
    &lt;% end %&gt;
  
    &lt;%= f.buttons do %&gt;
      &lt;%= f.commit_button 'Register as Artist' %&gt;
    &lt;% end %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>The problem is the :artist fields are not rendered.</p>
  
  <p>I've also tried <code>f.inputs :for =&gt; :artist do |a|</code>. </p>
  
  <p>For some reason, using <code>@user.build_artist</code> does not display the artist's fields in the form. If I try <code>@user.artist = Artist.new</code> I get an error, because it tries to save the Artist and validation fails. </p>
  
  <p>How should I initialize the Artist model so I get the benefit of formtastic generators in a nested form? (Note that <code>@user</code> here is not a <code>:new_record?</code>)</p>