I am using ActiveAdmin gem in my project.
I have 2 models using has_many through association. The database schema looks exactly the same as the example in RailsGuide
It's work for me (with chosen)
permit_params category_ids: [] form do |f| inputs 'Shop' do input :category_ids, collection: Category.all.collect {|x| [x.name, x.id]}, as: :select, multiple: true, input_html: { class: "chosen-input", style: "width: 700px;"} end f.actions end