title: Creating a has_many association in a hidden-field
body: <p>Say User has_many Things.  In a User form, I'd like a hidden_field that lets me create a relationship between this new User and a pre-existing Thing, say of id 8.  What's wrong with the following code snippet?  I think I'm just forgetting some syntax here.</p>
  
  <pre><code>&lt;% f.hidden_field 'things[]', :value =&gt; 8 %&gt;
  </code></pre>