Agile Web Development with Rails, Edition 4

10.2 Iteration E2: Handling Errors 9.4 Playtime

10.1 Iteration E1: Creating a Smarter Cart

Change the cart to track the quantity of each product.

Add a quantity column to the line_item table in the database.

rails generate migration add_quantity_to_line_items quantity:integer
      invoke  active_record
      create    db/migrate/20140204204151_add_quantity_to_line_items.rb

Modify the migration to add a default value for the new column

edit db/migrate/20140204204151_add_quantity_to_line_items.rb
class AddQuantityToLineItems < ActiveRecord::Migration
  def self.up
    add_column :line_items, :quantity, :integer, default: 1
  end
 
  def self.down
    remove_column :line_items, :quantity
  end
end

Apply the migration

rake db:migrate
mv 20140204204151_add_quantity_to_line_items.rb 20140204000004_add_quantity_to_line_items.rb
==  AddQuantityToLineItems: migrating =========================================
-- add_column(:line_items, :quantity, :integer, {:default=>1})
   -> 0.0194s
==  AddQuantityToLineItems: migrated (0.0195s) ================================

Create a method to add a product to the cart by either incrementing the quantity of an existing line item, or creating a new line item.

edit app/models/cart.rb
 
  def add_product(product_id)
    current_item = line_items.find_by_product_id(product_id)
    if current_item
      current_item.quantity += 1
    else
      current_item = line_items.build(product_id: product_id)
    end
    current_item
  end

Replace the call to LineItem.new with a call to the new method.

edit app/controllers/line_items_controller.rb
  def create
    product = Product.find(params[:product_id])
    @line_item = @cart.add_product(product.id)
    @line_item.product = product
 
    respond_to do |format|
      if @line_item.save
        format.html { redirect_to(@line_item.cart,
          :notice => 'Line item was successfully created.') }
        format.xml  { render :xml => @line_item,
          :status => :created, :location => @line_item }
      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @line_item.errors,
          :status => :unprocessable_entity }
      end
    end
  end

Update the view to show both columns.

edit app/views/carts/show.html.erb
<% if notice %>
<p id="notice"><%= notice %></p>
<% end %>
 
<h2>Your Pragmatic Cart</h2>
<ul>    
  <% @cart.line_items.each do |item| %>
    <li><%= item.quantity %> &times; <%= item.product.title %></li>
  <% end %>
</ul>

Look at the cart, and see that's not exactly what we intended

get /carts/1

Your Pragmatic Cart

  • 1 × Programming Ruby 1.9 & 2.0
  • 1 × Programming Ruby 1.9 & 2.0

Generate a migration to combine/separate items in carts.

rails generate migration combine_items_in_cart
      invoke  active_record
      create    db/migrate/20140204204155_combine_items_in_cart.rb

Fill in the self.up method

edit db/migrate/20140204204155_combine_items_in_cart.rb
  def self.up
    # replace multiple items for a single product in a cart with a single item
    Cart.all.each do |cart|
      # count the number of each product in the cart
      sums = cart.line_items.group(:product_id).sum(:quantity)
 
      sums.each do |product_id, quantity|
        if quantity > 1
          # remove individual items
          cart.line_items.where(product_id: product_id).delete_all
 
          # replace with a single item
          item = cart.line_items.build(product_id: product_id)
          item.quantity = quantity
          item.save!
        end
      end
    end
  end

Combine entries

rake db:migrate
mv 20140204204155_combine_items_in_cart.rb 20140204000005_combine_items_in_cart.rb
==  CombineItemsInCart: migrating =============================================
==  CombineItemsInCart: migrated (0.0936s) ====================================

Verify that the entries have been combined.

get /carts/1

Your Pragmatic Cart

  • 2 × Programming Ruby 1.9 & 2.0

Fill in the self.down method

edit db/migrate/20140204000005_combine_items_in_cart.rb
  def self.down
    # split items with quantity>1 into multiple items
    LineItem.where("quantity>1").each do |line_item|
      # add individual items
      line_item.quantity.times do 
        LineItem.create cart_id: line_item.cart_id,
          product_id: line_item.product_id, quantity: 1
      end
 
      # remove original item
      line_item.destroy
    end
  end

Separate out individual items.

rake db:rollback
==  CombineItemsInCart: reverting =============================================
==  CombineItemsInCart: reverted (0.0651s) ====================================
 
rake db:migrate:status
 
database: db/development.sqlite3
 
 Status   Migration ID    Migration Name
--------------------------------------------------
  down    20140204000005  Combine items in cart
   up     20140204000004  Add quantity to line items
   up     20140204000003  Create line items
   up     20140204000001  Create products
   up     20140204000002  Create carts
 
mv db/migrate/20140204000005_combine_items_in_cart.rb db/migrate/20140204000005_combine_items_in_cart.bak

Every item should (once again) only have a quantity of one.

get /carts/1

Your Pragmatic Cart

  • 1 × Programming Ruby 1.9 & 2.0
  • 1 × Programming Ruby 1.9 & 2.0

Recombine the item data.

mv db/migrate/20140204000005_combine_items_in_cart.bak db/migrate/20140204000005_combine_items_in_cart.rb
rake db:migrate
==  CombineItemsInCart: migrating =============================================
==  CombineItemsInCart: migrated (0.0937s) ====================================

Add a few products to the order.

post /line_items?product_id=2
You are being redirected. 
get http://localhost:3000/carts/1

Line item was successfully created.

Your Pragmatic Cart

  • 2 × Programming Ruby 1.9 & 2.0
  • 1 × CoffeeScript
post /line_items?product_id=3
You are being redirected. 
get http://localhost:3000/carts/1

Line item was successfully created.

Your Pragmatic Cart

  • 3 × Programming Ruby 1.9 & 2.0
  • 1 × CoffeeScript

Try something malicious.

get /carts/wibble

ActiveRecord::RecordNotFound in CartsController#show 

Couldn't find Cart with ID=wibble

Rails.root: /home/rubys/git/awdwr/edition4/work-192-30/depot

Application Trace | Framework Trace | Full Trace 
app/controllers/carts_controller.rb:16:in `show'

Request

Parameters: 

{"id"=>"wibble"}

Show session dump

Show env dump

Response

Headers: 

None

10.2 Iteration E2: Handling Errors 9.4 Playtime