Eager loading backreferences in ActiveRecord in Rails -

apparently in rails application activerecord running more sql queries like. have simple one-to-many relation. each package…

class package < activerecord::base   has_many :screenshots end 

…there several screenshots…

class screenshot < activerecord::base   belongs_to :package    def image_url     "/screenshots/#{self.package.name}/#{self.id}_#{size}.png"   end end 

my first simple task show index page packages , screenshots. each package want show screenshots. path screenshot image determined package name. let's example , fetch package record "3dchess" package:

pkg = package.includes(:screenshots).find_by_name('3dchess') 

as can see eager-load screenshots. activerecord runs these 2 sql queries:

  package load (1.4ms)  select "packages".* "packages" "packages"."name" = '3dchess' limit 1   screenshot load (2.1ms)  select "screenshots".* "screenshots" "screenshots"."package_id" in (243) 

i had expected done in 1 query don't mind two. when i'm displaying 20 screenshots on index page 20 additional sql queries. , turns out caused virtual attribute "image_url" of screenshot model.

what i'm doing in template:

<% @packages.each |package| %>   <li>     <% if package.screenshots %>       <%= image_tag package.screenshots.first.image_url %>     <% end %>   </li> <% end %> 

and each call of "image_tag package.screenshots.first.image_url" runs sql query like:

select "packages".* "packages" "packages"."id" = $1 order "packages"."id" asc limit 1 

so although i'm eagerloading screenshots when i'm loading package - not work other way round.

or put way - creates 2 sql queries mentioned above:

pkg = package.includes(:screenshots).find_by_name('3dchess') 

but when try access

pkg.package 

then activerecord runs additional query getting "package" information although should have them.

so question is: how can make activerecord eager-load backreference "screenshot" "package" automatically?