title: Rails - Render array attribute as string on failed validation
body: <p>I have a form field where a user inputs a string of tags <code>"foo, bar, foobar"</code>.  This is sent to the controller as <code>item_params[:tag_list]</code>.  In the controller my <code>create</code> action includes <code>@item = Item.create(tag_list: item_params[:tag_list])</code>.  This causes the tag_list attribute to be populated as an array of strings <code>["foo", "bar", "foobar"]</code>.  </p>
  
  <p>When validation fails I need the input field of the form to be populated with <code>"foo, bar, foobar"</code> and not the array of strings.  Is there a way to manipulate this before it goes back to populate the field of the form that failed validation?</p>