Rails - May 2008 - Perform custom calculation on model and sort by the calculation

Hi,

If anyone can point me in the right direction, I''d very appreciative.

I need to peform a calculation on a model (a number of columns) and
use a few columns of the model of the person who is logged in. Once
that calculation is done, I need to order the model by the resulting
calculation. Is there a way to this in the model class where I create
a temporary column on the fly and order by it?

Thanks,
Robert

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On 3 May 2008, at 19:24, Robert wrote:
>
> Hi,
>
> If anyone can point me in the right direction, I''d very
appreciative.
>
> I need to peform a calculation on a model (a number of columns) and
> use a few columns of the model of the person who is logged in. Once
> that calculation is done, I need to order the model by the resulting
> calculation. Is there a way to this in the model class where I create
> a temporary column on the fly and order by it?

SomeModel.find :all, :select => "*, a + b + c as  
some_calculation", :order => ''some_calculation''
(assuming you had columns a b c and were interested in their sum.

Fred

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Thanks.

I actually got it to work by reordering the array after the fact.  The
calculations needed were too involved and required a lot of case
switches to calculate so I wasn''t able to do it on initial query.

This works for me:
@SomeModel.sort! {|b,a| a.RankNumber <=> b.RankNumber }

It''s definitely not as efficient as doing it during the initial query,
but it works.



On May 3, 2:54 pm, Frederick Cheung
<frederick.che...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> On 3 May 2008, at 19:24, Robert wrote:
>
>
>
> > Hi,
>
> > If anyone can point me in the right direction, I''d very
appreciative.
>
> > I need to peform a calculation on a model (a number of columns) and
> > use a few columns of the model of the person who is logged in. Once
> > that calculation is done, I need to order the model by the resulting
> > calculation. Is there a way to this in the model class where I create
> > a temporary column on the fly and order by it?
>
> SomeModel.find :all, :select => "*, a + b + c as
> some_calculation", :order => ''some_calculation''
> (assuming you had columns a b c and were interested in their sum.
>
> Fred
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On Sat, May 3, 2008 at 10:09 PM, Robert
<robert-Pa/b8icLd6hBDgjK7y7TUQ@public.gmane.org> wrote:
>  I actually got it to work by reordering the array after the fact.  The
>  calculations needed were too involved and required a lot of case
>  switches to calculate so I wasn''t able to do it on initial query.
>
>  This works for me:
>  @SomeModel.sort! {|b,a| a.RankNumber <=> b.RankNumber }
>
>  It''s definitely not as efficient as doing it during the initial
query,
>  but it works.
There''s still a third option which is to precompute the rank (for
example in before filters) and have it stored in the database. That
way you can order by rank with straight SQL.

For example, the total of an invoice is computable from its parts, but
having it cached in the database makes sorting invoice tables by total
lighter.

Of course in your case this may not make sense, or it may not be worth
the trouble depending on the details.

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