Adding Cart items Ruby on Rails

Question

I am developing a very basic shopping cart and the issue I am having is that if I add more than one of the same product to my shopping cart I am not seeing the quantity increase and am instead just seeing multiple versions of the item.

For example I see:

1 x green light = £15 1 x green light = £15

rather than seeing:

2 x green light = £30

How can I make it so that my shopping cart will check for multiple versions in the cart and then add them together?

Currently I have:

Application Controller

  def current_cart
  if session[:cart_id]
    @current_cart ||= Cart.find(session[:cart_id])
  end
  if session[:cart_id].nil?
    @current_cart = Cart.create!
    session[:cart_id] = @current_cart.id
  end
  @current_cart
  end 

Cart Controller

   def show
    @cart = current_cart
   end

Cart Model

 has_many :items

 def total_price
   items.to_a.sum(&:full_price)
 end

Cart View

 <table id="line_items">
   <tr>
     <th>Product</th>
     <th>Qty</th>
     <th class="price">Unit Price</th>
     <th class="price">Full Price</th>
   </tr>

  <% for item in @cart.items %>
    <tr class="<%= cycle :odd, :even %>">
       <td><%=h item.product.name %></td>
       <td class="qty"><%= item.quantity %></td>
       <td class="price"><%= gbp(item.unit_price) %></td>
       <td class="price"><%= gbp(item.full_price) %></td>
    </tr>
   <% end %>
 <tr>

  <td class="total price" colspan="4">
    Total: <%= gbp(@cart.total_price) %>
  </td>
  </tr>
  </table>

FURTHER INFORMATION

Products#Index

  <%= link_to "Add to Cart", line_items_path(:product_id => product), :method => :post %>

Any advice people can offer would be much appreciated. Thanks!

New Setup - Causing Error Uninitialised Constant CartController

Routes.rb

 Checkout::Application.routes.draw do

   ActiveAdmin.routes(self)

   devise_for :admin_users, ActiveAdmin::Devise.config

   post '/add_to_cart/:product_id' => 'cart#add_to_cart', :as => 'add_to_cart'

   resources :carts
   resources :products
   resources :items

   root :to => 'products#index'

 end

Carts Controller

 class CartsController < ApplicationController

   def show
     @cart = current_cart
   end

   def add_to_cart
       current_cart.add_item(params[:product_id])
       redirect_to carts_path(current_cart.id)
   end

 end

Carts Model

class Cart < ActiveRecord::Base
has_many :items

def add_item(product_id)
        item = items.where('product_id = ?', product_id).first
    if item
        # increase the quantity of product in cart
        item.quantity + 1
        save
    else
        # product does not exist in cart
        product = Product.find(product_id)
        items << product
    end
    save
end

def total_price
    items.to_a.sum(&:full_price)
end
end

Product#Index

<table class="jobs">
    <thead>
        <tr>
            <th scope="col" id="name">Product Code</th>
            <th scope="col" id="company">Name</th>
            <th scope="col" id="company">Price</th>
            <th scope="col" id="company">Action</th>
        </tr>
    </thead>

    <tbody>
        <% @product.each do |product| %>
        <tr>    
            <td><%= product.product_code %></td>
            <td><%= product.name %></td>
            <td><%= gbp(product.price) %></td>
            <td><%= button_to "Add to Cart", add_to_cart_path(:product_id => product), :method => :post %></td>
        </tr>
        <% end %>
    </tbody>
</table>

Answer 1

In your Cart model, create a method called

def add_item(product_id)
  item = items.where('product_id = ?', product_id).first
  if item
    # increase the quantity of product in cart
    item.quantity + 1
    save
  else
    # product does not exist in cart
    cart.items << Item.new(product_id: product_id, quantity: 1)
  end
  save
end

In routes.rb,

post '/add_to_cart/:product_id' => 'cart#add_to_cart', :as => 'add_to_cart'

Change your Add to Cart route to a call add_to_cart method in the Cart controller.

def add_to_cart
  current_cart.add_item(params[:product_id])
  # redirect to shopping cart or whereever
end

This should give you the idea of what you are wanting to accomplish.