old-ribica/front-api/controllers/section.rb

get '/section' do
  Section.order(:name).all.to_json(:include =>
                                   [:categories => { :include => :sub_categories }])
end

get '/section/:id' do
  Section.find(params[:id].to_i).to_json(:include => [
  :categories => { :include => :sub_categories } ])
end