Agile Web Development with Rails, Edition 4

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/20121011071312_add_quantity_to_line_items.rb

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

edit db/migrate/20121011071312_add_quantity_to_line_items.rb
class AddQuantityToLineItems < ActiveRecord::Migration
  def change
    add_column :line_items, :quantity, :integer, :default => 1
  end
end

Apply the migration

rake db:migrate
mv 20121011071312_add_quantity_to_line_items.rb 20110711000004_add_quantity_to_line_items.rb
==  AddQuantityToLineItems: migrating =========================================
-- add_column(:line_items, :quantity, :integer, {:default=>1})
   -> 0.0058s
==  AddQuantityToLineItems: migrated (0.0059s) ================================

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
    @cart = current_cart
    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.json { render :json => @line_item,
          :status => :created, :location => @line_item }
      else
        format.html { render :action => "new" }
        format.json { render :json => @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
  • 1 × Programming Ruby 1.9

Generate a migration to combine/separate items in carts.

rails generate migration combine_items_in_cart
      invoke  active_record
      create    db/migrate/20121011071319_combine_items_in_cart.rb

Fill in the self.up method

edit db/migrate/20121011071319_combine_items_in_cart.rb
  def 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 20121011071319_combine_items_in_cart.rb 20110711000005_combine_items_in_cart.rb
==  CombineItemsInCart: migrating =============================================
==  CombineItemsInCart: migrated (0.3054s) ====================================

Verify that the entries have been combined.

get /carts/1

Your Pragmatic Cart

  • 2 × Programming Ruby 1.9

Fill in the self.down method

edit db/migrate/20110711000005_combine_items_in_cart.rb
  def 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
rake aborted!
An error has occurred, this and all later migrations canceled:
    
Can't mass-assign protected attributes: quantity

    
    
Tasks: TOP => db:rollback

    
(See full trace by running task with --trace)

    
==  CombineItemsInCart: reverting =============================================

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

    
get /carts/1

    



  
  

    
    


      

Your Pragmatic Cart


  • 2 × Programming Ruby 1.9
    • 



  


    

    

    
Recombine the item data.

    
rake db:migrate

    
Add a few products to the order.

    
post /line_items?product_id=2

    
    
      
  • 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
    • 


    
  • 1 × CoffeeScript
    • 



  


    

    

    
post /line_items?product_id=3

    
    
      
  • 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
    • 


    
  • 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-187/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