Question about eager loading associations and finder_sql

Rails
1

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

2

Chris H. wrote:

the bars table contains a column for storing cached data, which can
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.

3

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

Chris