title: Active Admin use related model fields in filters
body: <p>I have User model and Address, SocialProfile Model. I want to show the fields of Address model like "zip", "address_line_1" in the filter section of User resource in active admin. </p>
  
  <p>As Address model i want to show the fields of SocialProfile Model in same resource of User.
  how can I show the fields in both drop-down and text search in active admin of belongs_to model?   </p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    has_many :addresses
    has_many :social_profiles
  end
  
  class Address &lt; ActiveRecord::Base
   belongs_to :user
  end
  
  class SocialProfile &lt; ActiveRecord::Base
    belongs_to :user
  end
  </code></pre>
  
  <p><strong>Filters for User resource in app/admin/user.rb</strong></p>
  
  <pre><code>filter :mobile
  filter :full_name
  filter :zip #to use the address model's "zip" field
  filter :source #use social_profile model's "social" field           
  </code></pre>