title: Active Admin filter on two-level deep belongs_to association
body: <p>Say a <code>Person</code> has many <code>House</code> and <code>House</code> has many <code>Door</code>. (So <code>Door</code> has a <code>house_id</code> and House has a <code>person_id</code> field). I have a list of Doors in Active Admin, in which I want a filter that allows me to choose a person, and show me all doors in all of that person's houses.</p>
  <pre><code>ActiveAdmin.register Door do
    filter :knob_color
    filter :is_open
    filter :has_window
  
    filter :house_person      # DOESN'T WORK
    filter :house_person_name # DOESN'T WORK
  </code></pre>
  <p>The <code>Person</code> model table has a <code>name</code> field, so ActiveAdmin should be able to pick that up.</p>
  <p>How do I do this?</p>
  <h3>UPDATE</h3>
  <p>As requested, my situation rephrased in models.</p>
  <pre><code># id:   integer
  # name: string
  class Person &lt; ActiveRecord::Base
    has_many :houses
  end
  
  # id:        integer
  # person_id: integer
  class House &lt; ActiveRecord::Base
    belongs_to :person
    has_many   :doors
  end
  
  # id:         integer
  # house_id:   integer
  # knob_color: string
  # is_open:    boolean
  # has_window: boolean
  class Door &lt; ActiveRecord::Base
    belongs_to :house
  end
  </code></pre>