Rails - Sep 2006 - question about eager loading associations and finder_sql

I''ve got a situation where i have:

class Foo < ActiveRecord::Base
  has_many :bars
end

class Bar < ActiveRecord::Base
  belongs_to :foo
end

the bars table contains a column for storing cached data, which can
get quite large and there are occassions when I don''t want to load
that data when I instantiate an object, so I thought I might do

class Foo < ActiveRecord::Base
  has_many :bars
  has_many :bars_without_cache, :class_name => "Bar"
                  :finder_sql = ''select x, y, z from bars where foo_id
= #{self.id}''
end

only to find that if i eagar load the bars_without_cache association,
I still get the cache data I don''t want.  This makes sense in a way as
the whole point of eager loading is to load everything in one query so
of course the finder_sql is not being used. i should have figured
that.

so I guess I am asking is it possible, without having to resort to
find_by_sql or having to go to n+1 queries, to eager load an
association but only specific columns in the association?

Chris

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Chris Hall wrote:
> I''ve got a situation where i have:
> 
> class Foo < ActiveRecord::Base
>   has_many :bars
> end
> 
> class Bar < ActiveRecord::Base
>   belongs_to :foo
> end
> 
> the bars table contains a column for storing cached data, which can
> get quite large and there are occassions when I don''t want to load
> that data when I instantiate an object, so I thought I might do
> 
> class Foo < ActiveRecord::Base
>   has_many :bars
>   has_many :bars_without_cache, :class_name => "Bar"
>                   :finder_sql = ''select x, y, z from bars where
foo_id
> = #{self.id}''
> end
> 
> only to find that if i eagar load the bars_without_cache association,
> I still get the cache data I don''t want.  This makes sense in a
way as
> the whole point of eager loading is to load everything in one query so
> of course the finder_sql is not being used. i should have figured
> that.
> 
> so I guess I am asking is it possible, without having to resort to
> find_by_sql or having to go to n+1 queries, to eager load an
> association but only specific columns in the association?
You''ll have to use find_by_sql, and even so you''ll have to
install
the AR mod at http://mrj.bpa.nu/eager_custom_sql.rb so you can
eager load the bars.

You can prevent having to repeat yourself by defining a method like:

class Foo < ActiveRecord::Base

   def find_including_bars_without_cache(options = {})
     sql = ''...''
     sql << "where #{options[:conditions]}" unless
options[:conditions].nil?
     find_by_sql( sql, :include => :bars )
   end

end



-- 
We develop, watch us RoR, in numbers too big to ignore.


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thanks for the info.  I''ll look into that.

Chris

On 9/18/06, Mark Reginald James
<mrj-bzGI/hKkdgQnC9Muvcwxkw@public.gmane.org>
wrote:
>
> Chris Hall wrote:
> > I''ve got a situation where i have:
> >
> > class Foo < ActiveRecord::Base
> >   has_many :bars
> > end
> >
> > class Bar < ActiveRecord::Base
> >   belongs_to :foo
> > end
> >
> > the bars table contains a column for storing cached data, which can
> > get quite large and there are occassions when I don''t want to
load
> > that data when I instantiate an object, so I thought I might do
> >
> > class Foo < ActiveRecord::Base
> >   has_many :bars
> >   has_many :bars_without_cache, :class_name => "Bar"
> >                   :finder_sql = ''select x, y, z from bars
where foo_id
> > = #{self.id}''
> > end
> >
> > only to find that if i eagar load the bars_without_cache association,
> > I still get the cache data I don''t want.  This makes sense in
a way as
> > the whole point of eager loading is to load everything in one query so
> > of course the finder_sql is not being used. i should have figured
> > that.
> >
> > so I guess I am asking is it possible, without having to resort to
> > find_by_sql or having to go to n+1 queries, to eager load an
> > association but only specific columns in the association?
>
> You''ll have to use find_by_sql, and even so you''ll have
to install
> the AR mod at http://mrj.bpa.nu/eager_custom_sql.rb so you can
> eager load the bars.
>
> You can prevent having to repeat yourself by defining a method like:
>
> class Foo < ActiveRecord::Base
>
>    def find_including_bars_without_cache(options = {})
>      sql = ''...''
>      sql << "where #{options[:conditions]}" unless
options[:conditions].nil?
>      find_by_sql( sql, :include => :bars )
>    end
>
> end
>
>
>
> --
> We develop, watch us RoR, in numbers too big to ignore.
>
>
> >
>
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