I want to be able to drag and drag App model which is nested under Category model.

http://railscasts.com/episodes/196-nested-model-form-part-1

's the Railscast I've tried to follow.

Category controller

def move
  params[:apps].each_with_index do |id, index|
    Category.last.apps.update(['position=?', index+1], ['id=?', Category.last.id])
  end
  render :nothing => true
end

I'm able to sort Categories with something similar, but since I'm updating an attribute, I'm having trouble.

def sort
  params[:categories].each_with_index do |id, index|
  Category.update_all(['position=?', index+1], ['id=?', id])
end
  render :nothing => true
end

Any help is appreciated.