Ruby on Rails

hi everyone,

i have deployed a rails application on heroku
ruby version 1.9.3-p547
rails version 2.3.18

while i am using webrick server its working properly,but using unicorn server its not working ,that means it didn't support javascript, css and all,it shows error like this

ActionController::RoutingError (No route matches "/images/search.jpg" with {:method=>:get}):

so i have tried this command $ bundle exec rake assets:precompile RAILS_ENV=production RAILS_GROUPS=assets
but it shows error like this

rake aborted!
Don't know how to build task 'assets:precompile'

in rails 2.3.18 not even have assets.please help me to solve this.

thanks in advance