Associations

state.rb

ActiveAdmin.register State do

  index do
    column :name
    column "Associated Towns" do |state|
      ul do
        state.towns.each do |town|
         li link_to(town.name, admin_town_path(town))
        end
      end
    end
  end

  controller do
    def permitted_params
      params.permit state: :name
    end
  end

end

town.rb

ActiveAdmin.register Town do

  index do
    column :name
    column "State" do |town|
      town.state.each do |state|
        link_to(state.name, admin_state_path(state))
      end
    end
  end


  controller do
    def permitted_params
      params.permit town: [:name, :state_id]
    end
  end

end

Because towns only have one state, you don't need the loop from lines 6-8 in town.rb

Should just be able to call town.state.name instead of town.state.each ...