title: ActiveAdmin: Select nested Attributes based on the user's previous selection
body: <p>I would like to offer an easy interface for selecting information based on the hierarchy of the information.</p>
  
  <p>In my example the user needs to enter his county.</p>
  
  <p>A county is placed in a state and a state is part of a country. So this is my rails model:</p>
  
  <pre><code># Country
  class Country &lt; ActiveRecord::Base
    has_many :states
  end
  
  # State
  class State &lt; ActiveRecord::Base
    belongs_to :country
    has_many :counties
  end
  
  # County
  class County &lt; ActiveRecord::Base
    belongs_to :state
  end
  </code></pre>
  
  <p>So in my user interface the user should first select the country, based on the country selection the list of associated states is presented and based on the selected state the list of counties is presented.</p>
  
  <p>Is it possible to create this behaviour with ActiveAdmin</p>