Trick to achieve hierarchical view in activeadmin.

admin_section.rb

# encoding: utf-8

ActiveAdmin.register Section do
  config.filters = false
  
  index do
    column :name do |s|
      " #{ "――" * s.depth } #{s.name}"
    end
    default_actions
  end
  
  controller do
    def collection
      c = Section.hash_tree_flat
      c = Kaminari.paginate_array(c).page(params[:page])
      class << c
        def reorder(a=''); self end
        alias :count :size
      end
      c
    end
  end
end

section.rb

class Section < ActiveRecord::Base
  has_and_belongs_to_many :posts
  has_many :menu_items, as: :menuable
  acts_as_tree order: 'sort_order'

  def hash_tree_flat
    _ct_hash_tree_flat(hash_tree)
  end

  def _ct_hash_tree_flat(hash)
    hash.each_pair.map do |h_arr|
      [h_arr[0], _ct_hash_tree_flat(h_arr[1])] unless h_arr[1].nil?
    end
      .flatten
  end

end