Rails - Mar 2007 - Trying to understand something about instance variables.

My question is based on the following code from the agile book:

File: depot_f/app/controllers/store_controller.rb

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



File: depot_f/app/models/cart.rb

class Cart

֒attr_reader :items
def initialize
@items = []
end

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


File: depot_f/app/controllers/store_controller.rb

def add_to_cart
@cart = find_cart
product = Product.find(params[:id])
@cart.add_product(product)
end

Ok, so correct me if Im wrong, but dont instance variables only last
till the end of the method?

So how is it possible to keep appending items to the instance array
named @items?

Sorry for the newbie question, but I need to get this cleared up so I
can move on.

Thanks in advance

Kristen

-- 
Posted via http://www.ruby-forum.com/.

--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Ruby on Rails: Talk" group.
To post to this group, send email to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
To unsubscribe from this group, send email to
rubyonrails-talk-unsubscribe@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/rubyonrails-talk?hl=en
-~----------~----~----~----~------~----~------~--~---

> My question is based on the following code from the agile book:
>
> File: depot_f/app/controllers/store_controller.rb
>
> private
> def find_cart
> session[:cart] ||= Cart.new
>
> File: depot_f/app/models/cart.rb
>
> class Cart
>
> ֒attr_reader :items
> def initialize
> @items = []
> end
>
> def add_product(product)
> @items << product
> end
> end
>
>
> File: depot_f/app/controllers/store_controller.rb
>
> def add_to_cart
> @cart = find_cart
> product = Product.find(params[:id])
> @cart.add_product(product)
> end
>
> Ok, so correct me if Im wrong, but dont instance variables only last
> till the end of the method?
>
> So how is it possible to keep appending items to the instance array
> named @items?
>
> Sorry for the newbie question, but I need to get this cleared up so I
> can move on.
You''re right about instance variables.. but way up at the top is the
line:

   session[:cart] ||= Cart.new

This line checks to see if sesion[:cart] exists and if it doesn''t, it
sets
it to a new Cart.  Then it returns that cart object (either a new one or 
from the session).  This happens a bit farthe down in the line:

   @cart = find_cart

At which point you have a valid @cart which has as instance variables 
@items as they were saved in the session from the last request.

-philip

--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Ruby on Rails: Talk" group.
To post to this group, send email to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
To unsubscribe from this group, send email to
rubyonrails-talk-unsubscribe@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/rubyonrails-talk?hl=en
-~----------~----~----~----~------~----~------~--~---

Kristen,

I think that the statement, "Instance variables only last till the  
end of the method" is incorrect. Instance variables live as long as  
their containing object lives. So, the Cart object has an instance  
variable (an array) called @items. If your statement were true, there  
would be no point in adding anything to it because @items would  
disappear before anything useful can be done.

Here''s what''s going on starting with
store_controller#add_to_cart:

1. You''re creating a Cart object as a result of a find from the  
database and calling it @cart.
2. Next, a Product object is created as a result of a find from the  
database using an id parameter from the request. It''s only a local  
variable and it''s called product.
3. The product is added to the @cart.
4. The controller method ends. By default, Rails looks for a .rhtml  
file called /app/views/<controller>/<action>. By convention, store
is
the controller name, and add_to_cart is the action name.
5. Presumably, you want to show what''s in the cart in the view.  
That''s one reason why you''re using an instance variable-- to
make it
visible to the view. The local variable, product, is not available to  
the view.

Hope it helps,

-Anthony

On Mar 2, 2007, at 4:29 PM, Kristen wrote:
>
> My question is based on the following code from the agile book:
>
> File: depot_f/app/controllers/store_controller.rb
>
> private
> def find_cart
> session[:cart] ||= Cart.new
>
>
>
> File: depot_f/app/models/cart.rb
>
> class Cart
>
> ֒attr_reader :items
> def initialize
> @items = []
> end
>
> def add_product(product)
> @items << product
> end
> end
>
>
> File: depot_f/app/controllers/store_controller.rb
>
> def add_to_cart
> @cart = find_cart
> product = Product.find(params[:id])
> @cart.add_product(product)
> end
>
> Ok, so correct me if Im wrong, but dont instance variables only last
> till the end of the method?
>
> So how is it possible to keep appending items to the instance array
> named @items?
>
> Sorry for the newbie question, but I need to get this cleared up so I
> can move on.
>
> Thanks in advance
>
> Kristen
>
> -- 
> Posted via http://www.ruby-forum.com/.
>
> >

--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Ruby on Rails: Talk" group.
To post to this group, send email to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
To unsubscribe from this group, send email to
rubyonrails-talk-unsubscribe@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/rubyonrails-talk?hl=en
-~----------~----~----~----~------~----~------~--~---

> 2. Next, a Product object is created as a result of a find from the
> database using an id parameter from the request. It''s only a local
> variable and it''s called product.
> 3. The product is added to the @cart.
Hmm, Im kinda getting the idea now, but could you clarify the difference 
between an instance variable and a local variable.  How do I know when I 
should use an instance or a local variable.  Is there a rule of thumb?

Thanks you for your replies so far.

-- 
Posted via http://www.ruby-forum.com/.

--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Ruby on Rails: Talk" group.
To post to this group, send email to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
To unsubscribe from this group, send email to
rubyonrails-talk-unsubscribe-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
For more options, visit this group at
http://groups.google.com/group/rubyonrails-talk?hl=en
-~----------~----~----~----~------~----~------~--~---

An instance variable will be available to embedded ruby calls in your 
rhtml templates; a local variable will only be available within the method 
it''s defined in.

In the example from AWDWR you quoted:
> def add_to_cart
> @cart = find_cart
> product = Product.find(params[:id])
> @cart.add_product(product)
> end
@cart is an instance variable (denoted with an @ symbol), because it''s
an
instance of the Cart object. Product is a local variable, its scope is 
local to the action (method) add_to_cart. @cart can be exposed in web 
pages, but product cannot.

Further down AWDWR shows the instance variable @cart exposed in the 
add_to_cart.rhtml template:

<h1>Your Pragmatic Cart</h1>
<ul>
<% for item in @cart.items %>
<li><%= item.title %></li>
<% end %>
</ul>

The rule of thumb is: to make data visible to your user, use an instance 
variable. The AWDWR book does a much better job of illustrating these 
details.

Hope this helps!

Regards,
Dave
_______________________________
Information and Educational Technology
Kwantlen University College - 604-599-2120
"So powerful is the light of unity that it can illuminate the whole 
earth." --Bahá''u''lláh



Kristen <rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org> 
Sent by: rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
02/03/2007 06:14 PM
Please respond to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org


To
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
cc

Subject
[Rails] Re: Trying to understand something about instance variables.






> 2. Next, a Product object is created as a result of a find from the
> database using an id parameter from the request. It''s only a local
> variable and it''s called product.
> 3. The product is added to the @cart.
Hmm, Im kinda getting the idea now, but could you clarify the difference 
between an instance variable and a local variable.  How do I know when I 
should use an instance or a local variable.  Is there a rule of thumb?

Thanks you for your replies so far.

-- 
Posted via http://www.ruby-forum.com/.




--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Ruby on Rails: Talk" group.
To post to this group, send email to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
To unsubscribe from this group, send email to
rubyonrails-talk-unsubscribe@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/rubyonrails-talk?hl=en
-~----------~----~----~----~------~----~------~--~---

The decision about the visibility depends on the encapsulation principle in
object oriented programming. The variables that you declare must have as
minimal visibility as possible to get the  job done. The reason is that this
principle results in better quality software due to lower coupling.

You should read about basic OOP book if you are not familiar with basic
concepts. Here is a some good books: Object Oriented Software Construction
by Bertrand Meyer, Object Oriented Design Heuristics by Arthur J. Riel and
if you want a freebie, my book available for free at
http://guide.zepho.com/version1/<http://www.amazon.com/Object-Oriented-Software-Construction-Prentice-Hall-International/dp/0136291554/ref=pd_bbs_sr_1/104-8023659-2852731?ie=UTF8&s=books&qid=1172905283&sr=1-1>

On 3/2/07, Kristen
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org> wrote:
>
> Hmm, Im kinda getting the idea now, but could you clarify the difference
> between an instance variable and a local variable.  How do I know when I
> should use an instance or a local variable.  Is there a rule of thumb?
>
> Thanks you for your replies so far.
>
>
>
--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Ruby on Rails: Talk" group.
To post to this group, send email to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
To unsubscribe from this group, send email to
rubyonrails-talk-unsubscribe-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
For more options, visit this group at
http://groups.google.com/group/rubyonrails-talk?hl=en
-~----------~----~----~----~------~----~------~--~---

> @cart is an instance variable (denoted with an @ symbol), because
it''s
> an
> instance of the Cart object. Product is a local variable, its scope is
> local to the action (method) add_to_cart. @cart can be exposed in web
> pages, but product cannot.

I realy appreciate the tips guys.  There is one mroe thing that Im not 
quite clear about.

The method

def add_to_cart
@cart = find_cart
product = Product.find(params[:id])
@cart.add_product(product)
end

Is the @cart in @cart = find_cart the same @cart on the line 
@cart.add_product(product)?

I understand that the first line is either retrieving or creating a new 
object in the session.  So, is the third line just referencing that same 
@cart object or is it going into the model cart and invoking the 
add_product method?

Thank you!

-- 
Posted via http://www.ruby-forum.com/.

--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Ruby on Rails: Talk" group.
To post to this group, send email to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
To unsubscribe from this group, send email to
rubyonrails-talk-unsubscribe-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
For more options, visit this group at
http://groups.google.com/group/rubyonrails-talk?hl=en
-~----------~----~----~----~------~----~------~--~---