gistfile1.rb

ActiveAdmin.register Band do
  permit_params :name, :description, artists_attributes: [:id]
  

  # nested form to edit and new
  form do |f|
    f.inputs "Band" do
      f.input :name
      f.input :description
      f.input :artists, :as => :select, :input_html => { :multiple => true }

    end
    f.actions
  end
  
  controller do
    
    def create
      @band = Band.new(permitted_params[:band])
      @band.artists << Artist.find(params[:band][:artist_ids].drop(2))
      create!
    end
    
    def update
      @band.update(params[:band])
      update!
    end
    
  end
end