Rails - Jul 2008 - extra field as attribute in join table with has_many :through

hi everyone,
I''ve pasted some code over there http://pastie.org/230343 .

what I find strange is that the extra field in the join table called
"quantity" doesn''t shows up in the Drug resultset, even if I
added an
attr_accessor for good measure.

anybody knows why? I''m on edge rails.

thanks.

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On Jul 9, 2:37 am, Claudio Poli
<masterk...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> hi everyone,
> I''ve pasted some code over therehttp://pastie.org/230343.
>
> what I find strange is that the extra field in the join table called
> "quantity" doesn''t shows up in the Drug resultset, even
if I added an
> attr_accessor for good measure.
>
The attr_accessor would actually squash the attribute if it were
there.
It''s normal not to get the join table attributes - that''s just
the way
rails does things. Your workaround with the explicit select clause
should work, I expect that you are being fooled by ActiveRecord''s
display of your instance of Drug, which only displays attributes from
the drugs.table. Document.find(2).drugs.first.quantity should work
(but get rid of the attr_accessor first).

Fred


> anybody knows why? I''m on edge rails.
>
> thanks.
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Frederick Cheung wrote:
> On Jul 9, 2:37�am, Claudio Poli
<masterk...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> hi everyone,
>> I''ve pasted some code over therehttp://pastie.org/230343.
>>
>> what I find strange is that the extra field in the join table called
>> "quantity" doesn''t shows up in the Drug resultset,
even if I added an
>> attr_accessor for good measure.
>>
> The attr_accessor would actually squash the attribute if it were
> there.
> It''s normal not to get the join table attributes - that''s
just the way
> rails does things. Your workaround with the explicit select clause
> should work, I expect that you are being fooled by ActiveRecord''s
> display of your instance of Drug, which only displays attributes from
> the drugs.table. Document.find(2).drugs.first.quantity should work
> (but get rid of the attr_accessor first).
> 
> Fred
One way to guarantee the method exists,
and allow you to cast it to an integer would be;

class Drug < ActiveRecord::Base
  def quantity
    self[:quantity].to_i
  end
end

But I don''t like it,
because "quantity" isn''t actually a value of of the Drug,
it''s a value of the join.

clearly;
drug = document.drugs.first
drug.quantity != drug.reload.quantity (which will always be 0)

I''d prefer the following syntax;

drug = documents.drugs.first
quantity = documents.quantity_of_drug(drug)

class Document < ActiveRecord::Base
  def quantity_of_drug(drug)
    if join = join_for_drug(drug)
      return join.quantity
    else
      return 0
    end
  end

  private

  def join_for_drug(drug)
    self.document_drugs.detect {|dd| dd.drug == drug}
  end
end

however you''d need to ensure you loaded drugs through document_drugs,
rather than by a direct join
(not quite sure what rails does about this at the moment)
-- 
Posted via http://www.ruby-forum.com/.

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thanks for the suggestions, I''m going to do a bit of work on that as
advertised.
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