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Kristen

2007-Mar-04 02:53 UTC

Question about some code in the agile book

Hello,

My question is based on the following code form the 2ed agile book:

File: depot_g/app/models/cart_item.rb

class CartItem
attr_reader :product, :quantity
def initialize(product)
@product = product
@quantity = 1
end
def increment_quantity
@quantity += 1
end
def title
@product.title
end
def price
@product.price * @quantity
end
end


File: depot_g/app/models/cart.rb

def add_product(product)
current_item = @items.find {|item| item.product == product}
if current_item
current_item.increment_quantity
else
@items << CartItem.new(product)
end
end


My question is: how can this line:
current_item.increment_quantity
call a method from a different class?  The line is in cart.rb and its
calling a method from cart_item.rb.

Thanks in advance!

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dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org

2007-Mar-04 03:09 UTC

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Re: Question about some code in the agile book

Hi --

On Sun, 4 Mar 2007, Kristen wrote:
>
> Hello,
>
> My question is based on the following code form the 2ed agile book:
>
> File: depot_g/app/models/cart_item.rb
>
> class CartItem
> attr_reader :product, :quantity
> def initialize(product)
> @product = product
> @quantity = 1
> end
> def increment_quantity
> @quantity += 1
> end
> def title
> @product.title
> end
> def price
> @product.price * @quantity
> end
> end
>
>
> File: depot_g/app/models/cart.rb
>
> def add_product(product)
> current_item = @items.find {|item| item.product == product}
> if current_item
> current_item.increment_quantity
> else
> @items << CartItem.new(product)
> end
> end
>
>
> My question is: how can this line:
> current_item.increment_quantity
> call a method from a different class?  The line is in cart.rb and its
> calling a method from cart_item.rb.
current_item is an instance of class CartItem, and increment_quantity is
an instance method of CartItem.  It doesn''t matter what file
you''re
in.  The only thing that matters is whether or not the object
(current_item) understands the message you''re sending it
(increment_quantity).


David

-- 
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

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Kristen

2007-Mar-04 03:39 UTC

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Re: Question about some code in the agile book

> current_item is an instance of class CartItem, and increment_quantity is
> an instance method of CartItem.  It doesn''t matter what file
you''re
> in.  The only thing that matters is whether or not the object
> (current_item) understands the message you''re sending it
> (increment_quantity).

Hmm, isnt current_item a local variable?  It makes sense that in this 
code the current_item.increment_quantity is wokring, but my questions 
how it is working.  For me its essential to understand how it works so I 
know how to use in the future .  I would like to understand what is 
going on in the background.

Thanks.

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

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dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org

2007-Mar-04 13:02 UTC

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Re: Question about some code in the agile book

Hi --

On Sun, 4 Mar 2007, Kristen wrote:
>
>> current_item is an instance of class CartItem, and increment_quantity
is
>> an instance method of CartItem.  It doesn''t matter what file
you''re
>> in.  The only thing that matters is whether or not the object
>> (current_item) understands the message you''re sending it
>> (increment_quantity).
>
>
> Hmm, isnt current_item a local variable?  It makes sense that in this
> code the current_item.increment_quantity is wokring, but my questions
> how it is working.  For me its essential to understand how it works so I
> know how to use in the future .  I would like to understand what is
> going on in the background.
It''s not even the background -- it''s all up front :-)

current_item is a local variable which happens to refer to a CartItem
object.  You want that CartItem object to do something, so you send it
the message "increment_quantity".  The message-sending syntax in Ruby
takes the form:

   object.message

where object can be, and usually is, a variable.  "Sending a message"
is how you tell the object you want it to execute a particular method
(or trigger whatever unknown-method handlers it may have available to
it).

The "local" in "local variable" describes its scope.  Local
variables
inside method definitions are only in scope inside the definition:

   x = 1            # x in outer local scope
   def my_method
     x = 2          # x in method''s local scope
     puts x         # prints 2
   end
   puts x           # back to first x, so it prints 1

But, even though they are local in scope, they can have anything
assigned to them:

   class C
     def report
       puts "I''m a C instance!"
     end
   end

   def my_method
     x = C.new
     x.report
   end

   my_method     # I''m a C instance!

I definitely agree that it''s a good idea to get a good handle on Ruby
while you''re learning Rails.  There''s even a book written
exactly for
people who are trying to do exactly that :-)

David

-- 
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

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kirkr

2007-Mar-05 15:01 UTC

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Re: Question about some code in the agile book

Nice explanation David, you''re a helpful old chap.  And your book is
excellent.  I was a beta reader and it was/is a tremendous resource to
a Rails/Ruby developer

Good work mate

Keep it up

Kirk out

On Mar 4, 7:02 am, dbl...-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
wrote:> Hi --
>
> On Sun, 4 Mar 2007, Kristen wrote:
>
> >> current_item is an instance of class CartItem, and
increment_quantity is
> >> an instance method of CartItem.  It doesn''t matter what
file you''re
> >> in.  The only thing that matters is whether or not the object
> >> (current_item) understands the message you''re sending it
> >> (increment_quantity).
>
> > Hmm, isnt current_item a local variable?  It makes sense that in this
> > code the current_item.increment_quantity is wokring, but my questions
> > how it is working.  For me its essential to understand how it works so
I
> > know how to use in the future .  I would like to understand what is
> > going on in the background.
>
> It''s not even the background -- it''s all up front :-)
>
> current_item is a local variable which happens to refer to a CartItem
> object.  You want that CartItem object to do something, so you send it
> the message "increment_quantity".  The message-sending syntax in
Ruby
> takes the form:
>
>    object.message
>
> where object can be, and usually is, a variable.  "Sending a
message"
> is how you tell the object you want it to execute a particular method
> (or trigger whatever unknown-method handlers it may have available to
> it).
>
> The "local" in "local variable" describes its scope. 
Local variables
> inside method definitions are only in scope inside the definition:
>
>    x = 1            # x in outer local scope
>    def my_method
>      x = 2          # x in method''s local scope
>      puts x         # prints 2
>    end
>    puts x           # back to first x, so it prints 1
>
> But, even though they are local in scope, they can have anything
> assigned to them:
>
>    class C
>      def report
>        puts "I''m a C instance!"
>      end
>    end
>
>    def my_method
>      x = C.new
>      x.report
>    end
>
>    my_method     # I''m a C instance!
>
> I definitely agree that it''s a good idea to get a good handle on
Ruby
> while you''re learning Rails.  There''s even a book written
exactly for
> people who are trying to do exactly that :-)
>
> David
>
> --
> Q. What is THE Ruby book for Rails developers?
> A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
>     (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
> Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
> A. Ruby Power and Light, LLC (http://www.rubypal.com)

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dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org

2007-Mar-05 17:17 UTC

head link

Re: Question about some code in the agile book

Hi --

On Mon, 5 Mar 2007, kirkr wrote:
>
> Nice explanation David, you''re a helpful old chap.  And your book
is
> excellent.  I was a beta reader and it was/is a tremendous resource to
> a Rails/Ruby developer
>
> Good work mate
>
> Keep it up
Thanks -- I shall do my best!


David

-- 
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

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Rails - Mar 2007 - Question about some code in the agile book

Question about some code in the agile book

Re: Question about some code in the agile book

Re: Question about some code in the agile book

Re: Question about some code in the agile book

Re: Question about some code in the agile book

Re: Question about some code in the agile book