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dev

2010-May-05 19:21 UTC

Using self. Can you newbify this for me?

For some reason I am tripping up on the use of self and new.

class LineItem < ActiveRecord::Base
  belongs_to :order
  belongs_to :product

  def self.from_cart_item(cart_item)
    li = self.new
    li.product = cart_item.product
    li.quantity = cart_item.quantity
    li.total_price = cart_item.price
    li
  end
end

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Joshua Martin

2010-May-05 19:27 UTC

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Re: Using self. Can you newbify this for me?

I don''t know for sure because I haven''t encountered a
situation where I''d
need to do things like this... But I''ll give it a shot - I''m
open to
corrections..

class LineItem < ActiveRecord::Base
     belongs_to :order
     belongs_to :product

     def self.from_cart_item(cart_item)
         li = new
         li.product = cart_item.product
         li.quantity = cart_item.quantity
         li.total_price = cart_item.price
         return li
     end
end

You''d simply use the new method without the self because self is used
to
refer to the current instance of the object. Also, you should put return li
at the end; you don''t have to, but it just makes the code easier to
read.

On Wed, May 5, 2010 at 3:21 PM, dev
<devin.cormier-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> For some reason I am tripping up on the use of self and new.
>
> class LineItem < ActiveRecord::Base
>  belongs_to :order
>  belongs_to :product
>
>  def self.from_cart_item(cart_item)
>    li = self.new
>    li.product = cart_item.product
>    li.quantity = cart_item.quantity
>    li.total_price = cart_item.price
>    li
>  end
> end
>
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> .
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>
>

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Floyd Joseph

2010-May-05 20:14 UTC

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Re: Using self. Can you newbify this for me?

new is used to create a new object.

A new object is often created by using the class name followed by .new
e.g People.new, Items.new etc.

However in the example you gave you are creating a new object from
within the class, i.e. you are creating a LineItem object from within
the LineItem class and so instead of saying LineItem.new you use the
self keyword and it creates the object.

On 5 May, 19:21, dev
<devin.corm...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> For some reason I am tripping up on the use of self and new.
>
> class LineItem < ActiveRecord::Base
>   belongs_to :order
>   belongs_to :product
>
>   def self.from_cart_item(cart_item)
>     li = self.new
>     li.product = cart_item.product
>     li.quantity = cart_item.quantity
>     li.total_price = cart_item.price
>     li
>   end
> end
>
> --
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Marnen Laibow-Koser

2010-May-05 20:20 UTC

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Re: Using self. Can you newbify this for me?

dev wrote:> For some reason I am tripping up on the use of self and new.
> 
> class LineItem < ActiveRecord::Base
>   belongs_to :order
>   belongs_to :product
> 
>   def self.from_cart_item(cart_item)
>     li = self.new
>     li.product = cart_item.product
>     li.quantity = cart_item.quantity
>     li.total_price = cart_item.price
>     li
>   end
> end
This all looks correct.  What problem do you think you''re having? 
Which
tests are failing?

More importantly, why do you think you need to do this in the first 
place?  Although the method seems correct, it also seems like a strange 
thing for your app to need.

 Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org
-- 
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Robert Walker

2010-May-05 20:58 UTC

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Re: Using self. Can you newbify this for me?

dev wrote:> For some reason I am tripping up on the use of self and new.
> 
> class LineItem < ActiveRecord::Base
>   belongs_to :order
>   belongs_to :product
> 
>   def self.from_cart_item(cart_item)
>     li = self.new
>     li.product = cart_item.product
>     li.quantity = cart_item.quantity
>     li.total_price = cart_item.price
>     li
>   end
> end
This pattern looks very much like the NSCoding protocol pattern that
I''m
familiar with from the Cocoa frameworks. Compare what you have with this 
Objective-C example:

- (id)initWithCoder:(NSCoder *)coder {
    self = [super initWithCoder:coder];
    mapName = [[coder decodeObjectForKey:@"MVMapName"] retain];
    legendView = [[coder decodeObjectForKey:@"MVLegend"] retain];
    auxiliaryView = [coder decodeObjectForKey:@"MVAuxView"];
    magnification = [coder decodeFloatForKey:@"MVMagnification"];
    return self;
}

This implementation is slightly more abstract, and implemented as an 
instance method rather than a class method, but it serves essentially 
the same purpose.

To use this method you would do this:
id myObject = [[MyObject alloc] initWithCoder:coder];

Notice that Obj-C instance methods begin with a minus sign (-) and class 
methods begin with a plus sign (+). Since this is an instance method the 
instance must be allocated before being initialized. This alloc-init 
process is roughly equivalent to Ruby''s "new" method.

One way to declare a class method in Ruby is to begin it with "self." 
Which is what you are doing in your code (self.from_cart_item). In order 
to use class methods you send messages to the class rather than an 
instance of the class:

my_line_item = LineItem.from_cart_item # LineItem class is the receiver 
of message from_cart_item
-- 
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Rob Biedenharn

2010-May-05 21:06 UTC

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Re: Using self. Can you newbify this for me?

On May 5, 2010, at 3:21 PM, dev wrote:
> For some reason I am tripping up on the use of self and new.
>
> class LineItem < ActiveRecord::Base
>  belongs_to :order
>  belongs_to :product
>
>  def self.from_cart_item(cart_item)
>    li = self.new
>    li.product = cart_item.product
>    li.quantity = cart_item.quantity
>    li.total_price = cart_item.price
>    li
>  end
> end

While you already have some good answers from Joshua, Floyd, and
Marnen, I''ll add a bit to what they said from a Rails idiom
perspective.
(Or ActiveRecord idiom if you prefer.)

class LineItem < ActiveRecord::Base
   belongs_to :order
   belongs_to :product

   def self.from_cart_item(cart_item)
     new(:product => cart_item.product,
         :quantity => cart_item.quantity,
         :total_price => cart_item.price)
   end
end

I think I recognize that example, so I suspect that you''re working  
through the
Agile Wed Development with Rails.  Like others have said, you could  
have put
self.new since self is the default receiver of a method and from  
within a
class method (i.e., defined on the LineItem class itself rather than  
on an
instance of that class).  This is just a specialized form of instance  
creation
that knows how to create a new LineItem that has the properties from a
CartItem (which is presumably never in the database and thus not an
ActiveRecord model itself).

There are other ways of doing this same initialization, for example,  
with a block passed to new:

class LineItem < ActiveRecord::Base
   belongs_to :order
   belongs_to :product

   def self.from_cart_item(cart_item)
     new do |li|
       li.product = cart_item.product
       li.quantity = cart_item.quantity
       li.total_price = cart_item.price
     end
   end
end

You can think about LineItem.from_cart_item as a kind of cast or  
constructor if you''re coming from C or Java (but please let it me just
a fleeting thought that never appears once you "get it" from a Ruby  
perspective ;-).

-Rob

Rob Biedenharn
http://agileconsultingllc.com
	Rob-xa9cJyRlE0mWcWVYNo9pwxS2lgjeYSpx@public.gmane.org
http://gaslightsoftware.com
	rab-/VpnD74mH8+00s0LW7PaslaTQe2KTcn/@public.gmane.org

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Joe Smith

2010-May-06 18:37 UTC

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Re: Using self. Can you newbify this for me?

Joshua Martin wrote:>Also, you should put return li at the end; you
>don''t have to, but it just makes the code easier to read.
Oddly most ruby style guides I''ve seen disagree with this. They
recommend
using return only if one is returning from the middle of the function.



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Joshua Martin

2010-May-06 21:11 UTC

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Re: Re: Using self. Can you newbify this for me?

I understand their position, but don''t you think it''s kinda
weird to just
leave it like that... No operations or anything.. Ruby is all about
readability, and to just type the name of the object without performing any
operations on it is not really natural. It''s like, what the heck is it
doing
just sitting there like that? Why even waste the time typing it at all if it
doesn''t do anything?

Question: Doesn''t Ruby automatically return the object you were last
operating on? So, we could do whatever we want to the object we created, and
just let it return the object automagically?

On Thu, May 6, 2010 at 2:37 PM, Joe Smith
<unknown_kev_cat-PkbjNfxxIARBDgjK7y7TUQ@public.gmane.org>wrote:
>
> Joshua Martin wrote:
>
>> Also, you should put return li at the end; you
>> don''t have to, but it just makes the code easier to read.
>>
>
> Oddly most ruby style guides I''ve seen disagree with this. They
recommend
> using return only if one is returning from the middle of the function.
>
>
>
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Michael Pavling

2010-May-06 21:15 UTC

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Re: Re: Using self. Can you newbify this for me?

On 6 May 2010 22:11, Joshua Martin
<josmar52789-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> Question: Doesn''t Ruby automatically return the object you were
last
> operating on?
not quite.

it returns the result of the last operation.

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Rick DeNatale

2010-May-07 00:19 UTC

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Re: Re: Using self. Can you newbify this for me?

On Thu, May 6, 2010 at 5:15 PM, Michael Pavling
<pavling-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> On 6 May 2010 22:11, Joshua Martin
<josmar52789-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> Question: Doesn''t Ruby automatically return the object you
were last
>> operating on?
>
> not quite.
>
> it returns the result of the last operation.
Actually the value of the last expression.

Which is why ending a method with a variable name doesn''t  do nothing,
as Joshua seems to think, it provides the result of the method.

And it IS standard ruby practice.

-- 
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Michael Pavling

2010-May-07 07:47 UTC

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Re: Re: Using self. Can you newbify this for me?

On 7 May 2010 01:19, Rick DeNatale
<rick.denatale-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> On Thu, May 6, 2010 at 5:15 PM, Michael Pavling
<pavling-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> it returns the result of the last operation.
>
> Actually the value of the last expression.
Sorry; of course, expressions can be comprised of several operations :-)

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Rails - May 2010 - Using self. Can you newbify this for me?

Using self. Can you newbify this for me?

Re: Using self. Can you newbify this for me?

Re: Using self. Can you newbify this for me?

Re: Using self. Can you newbify this for me?

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Re: Using self. Can you newbify this for me?

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Re: Re: Using self. Can you newbify this for me?

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