title: Editing a has_one association in ActiveAdmin - avoid saving when nothing is entered
body: <p>I've got a model in which a very small percentage of the objects will have a rather large descriptive text. Trying to keep my database somewhat normalized, I wanted to extract this descriptive text to a separate model, but I'm having trouble creating a sensible workflow in ActiveAdmin. </p>
  
  <p>My models look like this:</p>
  
  <pre class="lang-rb prettyprint-override"><code>class Person &lt; ActiveRecord::Base
    has_one :long_description
  end
  
  class LongDescription &lt; ActiveRecord::Base
    attr_accessible :text, :person_id
    belongs_to :person
  
    validates :text, presence: true
  end
  </code></pre>
  
  <p>Currently I've created a form for editing the Person model, looking somewhat like this:</p>
  
  <pre class="lang-rb prettyprint-override"><code>  form do |f|
      ...
      f.inputs :for =&gt; [
                        :long_description,
                        f.object.long_description || LongDescription.new
                       ] do |ld_f|
        ld_f.input :text
      end
  
      f.actions
    end
  </code></pre>
  
  <p>This works for adding/editing the LongDescription object, but I still have an issue: I'd like to avoid validating/creating the LongDescription object if no text is entered.</p>
  
  <p>Anyone with better ActiveAdmin skills than me know how to achieve this?</p>