add auto complete in ActiveAdmin

admin_user.rb

ActiveAdmin.register AdminUser do
  # define routes for "autocomplete :admin_user, :email"
  collection_action :autocomplete_admin_user_email, :method => :get

  controller do
    autocomplete :admin_user, :email
  end

  index do
    column :email
    column :current_sign_in_at
    column :last_sign_in_at
    column :sign_in_count
    default_actions
  end

  # make "filter :email" can autocomplete
  filter :email_contains, :as => :autocomplete, :url => '/admin/admin_users/autocomplete_admin_user_email',
                          :label => "Search Email", :required => false, :wrapper_html => {:style => "list-style: none"}

  form do |f|                         
    f.inputs "Admin Details" do       
      f.input :email, :as => :autocomplete, :url => autocomplete_admin_user_email_admin_admin_users_path
      f.input :password
      f.input :password_confirmation
    end
    f.buttons
  end
end

I am getting this error

undefined method `email_contains' for #MetaSearch::Searches::Product:0xcf30280