Updated carts controller

cart.rb

class Cart < ActiveRecord::Base
  belongs_to :user
  has_many :cart_products
  has_many :products, :through => :cart_products

  def product_count
    cart_products.pluck(:quantity).sum
  end

  def add_product(product_id)
    if cart_product = cart_products.where(product_id).first
      cart_product.increment
      save
    else
      products << Product.find(product_id)
    end
  end

  def remove_product(product_id)
    if cart_product = cart_products.where(product_id: product_id).first
      cart_product.destroy
    end    
  end

  def update_quantity(product_id, quantity)
    if cart_product = cart_products.where(product_id: product_id).first
      cart_product.write_attribute(:quantity, quantity)
    end
  end
end

gistfile1.rb

class CartsController < ApplicationController

  def show
  end

  def update
    if current_cart.add_product(params[:product_id])
      redirect_to product_path(params[:product_id]), :notice => "Product has been added."
    end
  end
end

What an amazing gist.