title: How would you add Active Admin index columns from a sibling table?
body: <p>In my Customers_Rewards active admin I currently display columns from the Customer_Rewards and Customer data table, but I also want to display columns (such as store_id) from the Order data table.  </p>
  
  <pre><code>ActiveAdmin.register CustomerReward do
      menu :parent=&gt;"Customers", :priority=&gt;10, :label=&gt;"Activated Rewards"
  
    index do
      selectable_column
      column("Customer ID"){|u| u.customer.id }
      column("Store ID"){|u| u.order.store_id }  #DOES NOT WORK
      column("First name"){|u| u.customer.first_name }
      column("Last name"){|u| u.customer.last_name }
      column("Email"){|u| u.customer.email }
      column :reward
      column :points_redeemed
      column :expires_at
      column :redeemed_at
      bool_column :is_active
      actions
    end
   _____________________________________________________
  class CustomerReward &lt; ActiveRecord::Base
    belongs_to :customer
  end
  
  class Customer &lt; ActiveRecord::Base
    has_many :addresses, :order =&gt; 'is_active desc, is_default desc'
    has_many :customer_rewards
    has_many :orders
  end
  
  class Order &lt; ActiveRecord::Base
    belongs_to :customer
  end
  </code></pre>
  
  <hr>
  
  <pre><code>I get an error message of: 
   undefined method `order' for #&lt;Customer:0x007fb1dc2e3160&gt;
  </code></pre>