Rails - Jul 2006 - Calculate methods on attr_accessor objects

Is it possible to use the calculate methods (sum, etc.) on attr_accessor
attributes?

Ie:

Class Item < ActiveRecord::Base
  attr_accessor :total_cost

  def total_cost
    @total_cost = self.cost_unit * self.amount
  end

calling Item.sum(:total_cost) or Item.sum(''total_cost'') both
return a
unknown column ''total_cost'' mysql error.

I thought that active record considers attr_accessor objects the same as
columns in the table and all methods available to the model class would be
available to the attr_accessor too. Is that not so?


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On Thursday, July 27, 2006, at 11:02 AM, zer0halo wrote:
>Is it possible to use the calculate methods (sum, etc.) on attr_accessor
>attributes?
>
>Ie:
>
>Class Item < ActiveRecord::Base
>  attr_accessor :total_cost
>
>  def total_cost
>    @total_cost = self.cost_unit * self.amount
>  end
>
>calling Item.sum(:total_cost) or Item.sum(''total_cost'')
both return a
>unknown column ''total_cost'' mysql error.
>
>I thought that active record considers attr_accessor objects the same as
>columns in the table and all methods available to the model class would be
>available to the attr_accessor too. Is that not so?
>
>
>--
>"Impossible is nothing."
>
>
>_______________________________________________
>Rails mailing list
>Rails@lists.rubyonrails.org
>http://lists.rubyonrails.org/mailman/listinfo/rails
>
IIRC the AR Calculations feature actually lets the DB do the  
calculation, so it needs to understand how to do the calculation before  
it will work.

Something like this may work...

Item.sum(:select=>''(cost_unit + amount)'')

Not tested, so YMMV.

_Kevin
www.sciwerks.com

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Thanks, Kevin. That didn''t work, but it got me going in the right 
direction.

Through a bit of trial and error I discovered that contrary to what the 
AR rdoc says, count_by_sql accepts other methods than just COUNT.

So this works:

Item.count_by_sql("select sum(cost_unit * amount) from items")

Not very "rails like" though :-(. I should probably write a 
calculate_by_sql helper that accepts all the various calculate commands 
for those instances where you need more than just the sum of a single 
column or where that column doesn''t exist.

Kevin Olbrich wrote:
> Something like this may work...
> 
> Item.sum(:select=>''(cost_unit + amount)'')

-- 
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> Item.count_by_sql("select sum(cost_unit * amount) from items")
Item.sum ''cost_unit * amount''


-- 
Rick Olson
http://techno-weenie.net

Ah of course, I figured there was a simple Rails way but hadn''t found 
it. Why didn''t I try that? :-)

The kind of thing that would be handy to add to the documentation.

By the way, there used to be annotated documentation at 
rails.outertrack.com but it''s down these days. Has it moved elsewhere?
I
could add a comment on this to it.

Rick Olson wrote:
>> Item.count_by_sql("select sum(cost_unit * amount) from
items")
> 
> Item.sum ''cost_unit * amount''

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On Thursday, July 27, 2006, at 9:41 PM, François Montel
wrote:
>Ah of course, I figured there was a simple Rails way but hadn''t
found
>it. Why didn''t I try that? :-)
>
>The kind of thing that would be handy to add to the documentation.
>
>By the way, there used to be annotated documentation at
>rails.outertrack.com but it''s down these days. Has it moved
elsewhere? I
>could add a comment on this to it.
>
>Rick Olson wrote:
>>> Item.count_by_sql("select sum(cost_unit * amount) from
items")
>>
>> Item.sum ''cost_unit * amount''
>
>
>--
>Posted via http://www.ruby-forum.com/.
>_______________________________________________
>Rails mailing list
>Rails@lists.rubyonrails.org
>http://lists.rubyonrails.org/mailman/listinfo/rails
http://railsmanual.org

_Kevin
www.sciwerks.com

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