cart.rb

require 'sinatra'

enable :sessions

@@products = {
  1 => {name: "Lotion", price: 9.99, weight: 4},
  2 => {name: "Bar Soap", price: 4.99, weight: 4},
  3 => {name: "Liquid Soap", price: 7.99, weight: 4}
}

get '/cart' do
  product_id = params[:product_id]
  action = params[:action]

  session[:cart] ||= {}

  case action
  when 'add'
    if @@products[product_id]
      unless session[:cart].key?(product_id)
        session[:cart][product_id] = 0
      end
      session[:cart][product_id] += 1
    end
  when 'update'
    if params[:quantity].is_a?(Hash)
      params[:quantity].each do |product_id, quantity|
        if @@products[product_id] && session[:cart].key?(product_id)
          session[:cart][product_id] = quantity
        end
      end
    end
  when 'remove'
    if session[:cart].key?(product_id)
      session[:cart].delete(product_id)
    end
  end

  session[:cart].inspect
end