The Depot "Add to Cart" sessions question (2.ed agile book)

Rails
1

Hello,

Im in the process of working my way through the 2.ed of the agile book.
As many of you already know, the book goes through the regular
development cycle of the Depot application. I have hit a wall
when they started talking about the sessions and the exact mechanics of
how an item is added to the session.

The code looks like this:

depot/app/controllers/store_controller:
def add_to_cart
@cart = find_cart
product = Product.find(params[:id])
@cart.add_product(product)
end

private
def find_cart
session[:cart] ||= Cart.new
end
end

depot/app/models/cart.rb
class Cart
attr_reader :items

def initialize
@items = []
end

def add_product(product)
@items << product
end
end

depot/app/views/store/add_to_cart.rhtml

Your Pragmatic Cart

    <% for item in @cart.items %>
  • <%= h(item.title) %>
    • <% end %>

So, this is what I understand what is happening:
When the “add to cart” button is clicked, the add_to_cart method is run
and a hash is either retrieved or created (not sure exactly how this
work). Does Cart.new mean that it goes into the model Cart and creates
a new item? Does this also mean that the initialize method inside the
Cart model is run?

Also, the line product = Product.find(params[:id]) How come they arn’t
using an @ sign infront of the product?

What are the mechanics of the line @cart.add_product(product)?

The line @cart.items, is the items referring to the line attr_reader
:items?

When is the initialize method in the model used?

Any help on this topic would be greatly appreciated! Thanks in advance!

2

Hi Kristen, please see my answers to your questions below. Also, I
would recommend the you read:

Programming Ruby Chapters 1 - 7

or

Ruby For Rails

Good luck,

-Conrad

On 2/6/07, Kristen [email protected] wrote:

[snip]

So, this is what I understand what is happening:
When the “add to cart” button is clicked, the add_to_cart method is run
and a hash is either retrieved or created (not sure exactly how this
work). Does Cart.new mean that it goes into the model Cart and creates
a new item? Does this also mean that the initialize method inside the
Cart model is run?

Actually, Cart.new creates an instance of the Cart class. For example,

@cart = Cart.new

Now, when ‘new’ is called on the Cart class, the Cart’s initialize
method is called. This intern initializes the items array to be empty
(i.e. []). Then the new instance is returned and assigned to @cart in
the example above.

Also, the line product = Product.find(params[:id]) How come they arn’t
using an @ sign infront of the product?

The reason for this is that find is a class method. Thus, we invoke
class methods
using the following syntax:

class_name.method_name

e.g. Product.find

If it were an instance method, then we would have something similar to
the following:

@product.name # where name is an attribute accessor defined on the class
Product

What are the mechanics of the line @cart.add_product(product)?

@cart is an instance of the Cart class and we’re passing a message to
this instance called add_product which takes product instance as a
parameter. In short, we’re adding a product to the @cart instance.

The line @cart.items, is the items referring to the line attr_reader
:items?

Yes, this is correct.

When is the initialize method in the model used?

It is invoked anytime that you call new on a class. For example,

@cart = Cart.new