active admin sort by association

gistfile1.rb

### Model
class ModelA < ActiveRecord::Base
	has_many :modelb
end

class ModelB < ActiveRecord::Base
	belongs_to :modela
	has_many :modelc
end

class ModelC < ActiveRecord::Base
	belongs_to :modelb
	has_one :modela, :through => :modelb
end

### active admin side
ActiveAdmin.register ModelC do

	scope :joined, :default => true do |modelcs|
		modelcs.includes [:modelb]
		modelcs.includes [:modela]
	end

	index do
		column :modelb, :sortable => :'modelb.title' do |modelc|
			link_to modelc.modelb.title, admin_modelb_path(modelc.modelb) unless modelc.modelb.nil?
		end
		column :modela, :sortable => :'modela.title' do |poi|
			link_to modelc.modelb.modela.title, admin_park_path(modelc.modelb.modela) unless modelc.modelb.nil? || modelc.modelb.modela.nil?
		end
	end

Any idea? It doesn't work for me. =/
Thanks at all.