How would you add Active Admin index columns from a sibling table?

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.

ActiveAdmin.register CustomerReward do
    menu :parent=>"Customers", :priority=>10, :label=>"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 < ActiveRecord::Base
  belongs_to :customer
end

class Customer < ActiveRecord::Base
  has_many :addresses, :order => 'is_active desc, is_default desc'
  has_many :customer_rewards
  has_many :orders
end

class Order < ActiveRecord::Base
  belongs_to :customer
end

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I get an error message of: 
 undefined method `order' for #<Customer:0x007fb1dc2e3160>