Nested categories with Active Admin

categories.rb

ActiveAdmin.register Category do
  extend CollectiveIdea::Acts::NestedSet::Helper

  form do |f|
    f.inputs do 
      f.input :name
      f.input :parent_id, :as => :select, :collection => (nested_set_options(Category, @category) {|i, level| "#{'-' * level} #{i.name}" })
    end
    f.buttons
  end

end

This didn't work for me. I had to use

f.input :parent_id, , :as => :select, :collection => (nested_set_options(Category, @category) {|i| "#{'-' * i.level} #{i.name}" } )