Grouped table index for ActiveAdmin

application.rb

# config/application.rb
module MyApp
  class Application < Rails::Application
    config.autoload_paths << config.root.join('lib')
  end
end

book.rb

# models/book.rb
class Book < ActiveRecord::Base
  belongs_to :shelf
  
  def shelf_name
    shelf.name
  end
end

books.rb

# admin/books.rb
ActiveAdmin.register Book do
  index as: :grouped_table, group_by_attribute: :shelf_name do
    # columns
  end
end

index_as_grouped_table.rb

# lib/active_admin/views/index_as_grouped_table.rb
require 'active_admin/views/index_as_table'

module ActiveAdmin
  module Views
    class IndexAsGroupedTable < IndexAsTable

      def build(page_presenter, collection)
        if group_by_attribute = page_presenter[:group_by_attribute]
          collection.group_by(&group_by_attribute).sort.each do |group_name, group_collection|
            h3 group_name
            super page_presenter, group_collection
          end
        else
          super
        end
      end

    end
  end
end