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Jeff

2006-Oct-11 14:27 UTC

Syntax for calling the has_many class methods

I have a simple Active Record model:

class Order < ActiveRecord::Base
  has_many  :line_items
  def add_line_items_from_cart(cart)
    cart.items.each do |item|
      li = LineItem.from_cart_item(item)
      line_items << li
    end
  end
end

This syntax works fine:
 line_items << li

This also works:
 self.line_items << li

But this syntax fails with an error (undefined method `line_items'' for
Order:Class):
 Order.line_items << li

Why the error?

Thanks,
Jeff


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Michael Campbell

2006-Oct-11 14:41 UTC

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Re: Syntax for calling the has_many class methods

On 10/11/06, Jeff <objectis-/E1597aS9LQAvxtiuMwx3w@public.gmane.org>
wrote:>
> I have a simple Active Record model:
>
> class Order < ActiveRecord::Base
>   has_many  :line_items
>   def add_line_items_from_cart(cart)
>     cart.items.each do |item|
>       li = LineItem.from_cart_item(item)
>       line_items << li
>     end
>   end
> end
>
> This syntax works fine:
>  line_items << li
>
> This also works:
>  self.line_items << li
>
> But this syntax fails with an error (undefined method `line_items''
for
> Order:Class):
>  Order.line_items << li
>
> Why the error?

I think because you''re trying to call something on the Order /class/,
rather than on an instance OF an Order.

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Jason Roelofs

2006-Oct-11 14:45 UTC

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Re: Syntax for calling the has_many class methods

#line_items is an instance methods. You can''t call them on a Class
because
there is no link between the models. Try to think of it through the
resulting SQL queries.

self.line_items -- find all line_items with a order_id of self.id

You have to think of what ''Order.line_items << li''
means. You can''t do
Order.line_items because that would be like saying "find all line_items
with
order_id of Class.id" which is obviously wrong. Line_items are linked to
Orders and so you need to have an order to find line_items.

Just keep in mind that all method calls on ActiveRecord objects are in fact
queries to the database (or cached queries).

Hope that helped.

Jason

On 10/11/06, Jeff <objectis-/E1597aS9LQAvxtiuMwx3w@public.gmane.org>
wrote:>
>
> I have a simple Active Record model:
>
> class Order < ActiveRecord::Base
>   has_many  :line_items
>   def add_line_items_from_cart(cart)
>     cart.items.each do |item|
>       li = LineItem.from_cart_item(item)
>       line_items << li
>     end
>   end
> end
>
> This syntax works fine:
> line_items << li
>
> This also works:
> self.line_items << li
>
> But this syntax fails with an error (undefined method `line_items''
for
> Order:Class):
> Order.line_items << li
>
> Why the error?
>
> Thanks,
> Jeff
>
>
> >
>

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Jeff

2006-Oct-11 15:42 UTC

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Re: Syntax for calling the has_many class methods

I get it, thanks.  I misunderstood the API documentation.  It explains
that has_many, et al are class methods.  Then it uses a diagram syntax
like this to describe the methods that has_many creates:

Firm#clients<<

Class#method is a syntax used to illustrate class methods in the
PickAxe book.  So I assumed clients<< was a class method, when in fact
it is an instance method.

Thanks again,
Jeff


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dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org

2006-Oct-11 15:52 UTC

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Re: Syntax for calling the has_many class methods

Hi --

On Wed, 11 Oct 2006, Jeff wrote:
>
> I get it, thanks.  I misunderstood the API documentation.  It explains
> that has_many, et al are class methods.  Then it uses a diagram syntax
> like this to describe the methods that has_many creates:
>
> Firm#clients<<
>
> Class#method is a syntax used to illustrate class methods in the
> PickAxe book.  So I assumed clients<< was a class method, when in
fact
> it is an instance method.
The #method notation is actually for an instance method, like
String#split.  Class methods are usually indicated like this:
ActiveRecord::Base.has_many.

But if the class whose instance method it is is Class, you''d get
Class#method....  So, for example, it''s correct to refer to
Class#new, since new is an instance method of class.


David

-- 
                   David A. Black |
dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
Author of "Ruby for Rails"   [1] | Ruby/Rails training &
consultancy [3]
DABlog (DAB''s Weblog)        [2] | Co-director, Ruby Central, Inc.  
[4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org

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Jeff

2006-Oct-11 16:02 UTC

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Re: Syntax for calling the has_many class methods

Thanks David.  You''re right.  The PickAxe does use Class#method to
describe an instance method.  I''m mentally challenged today!

-- Jeff


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Rails - Oct 2006 - Syntax for calling the has_many class methods

Syntax for calling the has_many class methods

Re: Syntax for calling the has_many class methods

Re: Syntax for calling the has_many class methods

Re: Syntax for calling the has_many class methods

Re: Syntax for calling the has_many class methods

Re: Syntax for calling the has_many class methods